Nah, I have to strongly disagree.
This imo is a slippery slope for Player B. Infact in Sins ive noticed it becomes soo slippy that a single foothold quickly develops into a 2-3 hour mopup operation where the game is already won and theres no challenge.
This happens because the AI is unable to defend itself with the current build.
After you destroy the AIs main fleet, the AI is effectively defeated. As for a human player, she would be able to rebuild it's fleet in some minutes if she has planned correctly before.
Also, the example you've showed above is very one sided. Let's look at a bit different situation:
Player A has 15 colonies, while player B has 10 colonies, since A is a better colony rusher then B. Both of them have equal culture on their colonies. Now B is a better strategist then A so he manages to attack an enemy colony and destroy it. Now because of the culture there he's unable to claim it for himself.
Now A has still 14 colonies left, while B didn't gain anything. Because of that, A can replace his losses easier then B, which has a smaller resource base than A.
The effect your described system now has, is, that someone with a lower amount of colonies will have a very hard time to match up to someone with a larger amount, since he just can't conquer those enemy colonies. Subsequently it gives a good advantage to the player with more colonies while it penalizes one with less. This is just the opposite effect of what you've wanted to achieve.
So, as a conclusion, the current system is bad for someone, who is on the losing (less property) side.
