Asteroids are the traditional "walls in space" within this genre. It's an arcade game, not a starship simulator. I had suggested a "hyperspace bubble" as an alternative, an "interdiction field", but that is limiting in some ways as well. If we made real warships in space, most of their weapons would be facing backwards. If we have equal ships except that your weapons face forwards and mine face backwards, you almost can't win that fight.
This flies in the face of historical and physical realities.
20th-century warships and fighter aircraft have almost invariably had more of their offensive armament facing forward than backwards. The invention of the synchronisation gear that allowed a machine gun to be fired forward through the propeller threatened to give Germany air supremacy in World War 1 until the Allies figured out their own methods of equipping fighters with forward-firing machine-guns, and experiments since with fighters firing their main armament in other directions, such as the British Defiant in WW2, were all failures. (The Germans did have success with upwards-firing Schrage Musik installations, but this was a supplement to the forward-firing armament, not the primary weapon.) Warships generally had the greatest firepower in broadsides due to practical limits imposed on hull design, but designers still try to pack the majority of the primary armament to also be able to fire forward if they can. Even tanks are generally designed to fight an enemy in the direction of their primary movement (which is what I would define as 'forward' for armoured vehicles that are capable of reversing): exceptions, such as the Archer, are also imposed by technical limitations, and for the Archer it was highly impractical to be firing its main gun and moving simultaneously (the gun would recoil into the driver's space...).
I really don't see considerations for space combat overriding the advantages of having your weapons facing in your primary direction of travel. In fact, if anything, they'd be stronger: in terrestrial combat, velocities are limited by various drag factors that are largely proportional to the velocity relative to the common reference frame of Earth. In space, this isn't the case. Any weapon fired by either ship will have the velocity of the firing ship in addition to the velocity it would have when fired from a stationary ship: in practice, this means that firing backwards offers no advantage in effective range or time to target over firing forwards to compensate for the practical complications of not being able to shoot at a target you're flying at. One possible advantage to firing backward at a pursuing opponent rather than firing forward at a pursued opponent is that it might complicate the pursuer's ability to engage in evasive action without breaking off the chase, but when ships are powered by reaction mass thrusters of some description (as seems to be the case in Star Control) this may be counterbalanced by the fact that those thrusters are probably more difficult to 'harden' against enemy fire.
So, history (particularly aeronautical history, which is what space combat physics are likely to be closest to) has shown that forward-facing armaments are superior, and if anything the physics of space combat appear to reinforce this. (Caveat: In situations where engines are relatively weak compared to gravity and ships mostly operate by orbital mechanics, pointing a ship directly towards an enemy ship might not be the best way of getting closer to it, and in these circumstances ships would probably want weapons able to cover as many directions at once rather than focusing forward. However, this does not appear to be the case in the majority of any Star Control battle map)
Now, you may be thinking in terms of Star Control physics, where projectiles do not necessarily gain the velocity of the firer (they do in some cases, such as the Kohr-Ah FRIED, but there are also some very clear cases where they do not such as the Mycon plasmoids). Even here, though, game experience does not seem to hold to your hypothesis: the Spathi Eluder, while a good ship, is not unbeatable, despite having high speed and agility, above-average durability, and a good rear-firing weapon. In fact, the statistics of the Eluder ameliorate some of the disadvantages coming from having their primary weapon: the Eluder can turn quickly allowing it to move into firing position, turn around to point its rear at the enemy, and commence engaging in the Spathi's second-favourite tactic (their favourite, of course, being not fighting in the first place). Ships with slower turn rates would not be able to pull this off as efficiently.
The barrier is needed because the fight itself is unrealistic. Nobody would ever fight to the death in the middle of empty space for no reason at all. Your natural instinct is to stay away, and safe, as soon as you start to lose or have a significant disadvantage. So that is what most people will do. It isn't only about "refusal to engage", there really is a lot too it. For example, as you again mentioned the ships can be designed to work on an open map. The Mycon works on an open map. But if you rely on ship design then your ships will all be very much the same. No fast ships with long range weapons, no slow ships with short range weapons... and those are just the two most obvious restrictions.
And in the SC2 adventure mode, this was represented by being able to warp out as long as you had a few seconds where you could do so, allowing for ships to be evacuated from unfavourable matchups... at the cost of not being available for the rest of the battle.
Melee is a bit 'gameified' in the sense that there would be no advantage to the player in warping out over going out in a blaze of glory - the ship is gone for the rest of the map regardless. Unless one player decides to troll the other by dragging out the match, this works: the player with an unwinnable matchup just does what damage they can before bringing up another ship. My proposal, however, would allow for such trolling to be punished (by assuming that a ship that does nothing but run away will eventually just warp-out) without throwing out the baby with the bathwater by turning the simulated freedom of the wraparound map into an unrealistic cage-match. (Having an objective - most likely the planet - would also serve to limit keep-away behaviour.)
An objective removes the need for a barrier. The barrier is only needed when the only objective is attrition, destroying the other ship. But in a 1v1 duel in open space, that just won't work without containing the faster ship or designing all of the ships to work on an open map. Designing all of the ships to work on an open map makes them all pretty much the same, there would be very little variety.
The Star Control 2 ships don't feel like they're all the same to me. In fact, they appear more distinct from one another than I've seen from the SC:O ships thus far, although that may be experience talking.
Faster ships generally having shorter-range weapons, to me, is something that makes sense from a realism perspective as well as a gameplay sense. Longer-range weapons are likely to be bigger. Bigger weapons require more space to be mounted on a ship, which in turn requires bigger ships. Bigger ships are likely to be slower.
Furthermore, the SC2 model still allows for long-range weapons on faster ships, since it also has a number of slower ships that are able to protect themselves against being plinked at by such weapons, such as point-defence systems, shields, and other defences, while often still having a means of shooting back. While ships that are slow and have short-range weapons, such as the Avatar and Intruder, have means of reducing the mobility of the enemy ship.
EDIT: I should also mention that the current map is about the smallest "boxing ring" you would ever want to use. I would think the "standard map" would be about 50% bigger than the current one. The current map is so small that the contracting ring really doesn't even make much sense on this map. Larger maps might have that ring, that contracts down to about the size of the current map. The current map, with the planet and gravity removed (and I still like just a straight "energy barrier" wall in space better than the asteroids) is really a "serious tournament map". A "standard map" might be about 50% bigger than this, and a "large map" might be twice this size. These larger maps might contract after a time limit, but the current "small map" doesn't even need the contracting ring. The current map is very small, and the planet/gravity at the center making it even smaller and denying the center to the slower ship. The planet/gravity is hurting the slower ships a lot right now, it is taking up the space they want to control without providing a slingshot to give them something back in return for that, which is probably a part of why none of the ships are actually slow.
I've got to say that I find this to be somewhat irrelevant. Less of a bad thing may be less bad, but this does not make it a good
I'd agree that, from a believability perspective, an artificial energy field would make more sense than an unnatural asteroid field, but at the bottom line, to me it's less enjoyable than the original formula. And from what I've seen of other people's feedback, that appears to be the majority opinion: there are a few people praising the restricted map, but a lot more saying that they prefer the original wraparound style.
Given that SC2 appears to have managed a high degree of variation between ships despite having the wraparound map that you claim limits ship design, it appears that the only real advantage to the cage-match play is to limit trolling behaviour - and I think there are ways to do that which doesn't punish everybody for the sake of the odd troll.