Well the alternative is leaving the AI as a one city civ, getting energy per turn as tribute and getting powerful agreements from them again. Rinse and repeat with every civ. That would be a strong game exploit and if the devs let it in, we would be hearing cries to remove it. Now I can't speak for the devs but maybe they just wanted to try to remove that exploit?
I was saying that too. But, I no longer "fully" believe in that, since if the losing civ peaces out and the attacker lets them, the attacker is only hurting themselves a bit. That civ is still alive and may have good relations with your competitors and continues to feed them bonuses. Plus, I learned some other things:
1) At sanctioned, I don't think you can trade traits, along with other things.
2) It looks like leaders are limited to gaining only 5 additional traits from other leaders, is this correct?
3) Leaders can always deny a trait deal proposal, even when allies. This one surprised me, but I like it, it's not only a matter of having the DC to buy them.
So, I think factions should still try to survive and peace out. But, the attacker shouldn't be able to get higher than sanctioned with that leader for a long time, as punishment, before you can start repairing relations with them and trade with them again for bonuses.
I think this would give a good balance to either exterminating them for good and also removing any bonuses from your competitors, OR letting them live with a few cities for the chance to trade with them again if you're willing to back off of them and wait for a while.
Personally, I still don't want the inferior trading table back. But I also don't want cities as the only bargaining chip - that kind of sucks and you often will get crap cities. Players would often rather get nothing than that, since you're already satisfied with what you've conquered. Plus, you can't gain anything from smaller skirmishes won. Finally, giving up cities messes with the AI accepting peace, if they're willing to at the time.
In the other thread I suggested a Spoils of War system, which I think could improve the War Score system if it was implemented in some form and polished. You're rewarded for the score you accumulate and for simply winning, it makes it more competitive.
(written on my phone, hope it makes sense!)