[Q]

I agree it should be easier to end a war and the AI should propose even a trade of a treaty against e.g. a system (depending if the AI lost or won the Last few combats).
But it's not completely impossible now. If you are at war with the AI, you don't fullfil the Mega empire settings and avoid all hostility, you might be able to get a new treaty after a certain time. There is a miminum of turns after a war breaks out, the AI won't accept any treaty (it's specified in the political text-file IIRC). But after that it's possible and I've seen it more than once.