I dunno if this has been suggested.  But anyway, I was thinking maybe for a "quicker" game, make an option that if a player has no planets left, he/she loses the game.

Or perhaps if a player has no planets AND no unoccupied planets that can be immediately colonized, then he/she loses.

The idea is that if you can't pay for fleet upkeep (because you have no planets and no income), well, you get no fleet.