The war length factors into it accepting a peace deal as does the might be purged modifier etc but that isn't the same as having to get 100% warscore to just humiliate you're being ridiculous.
What I meant to say was, that the AI will not accept peace, if you just started a war. Some time needs to pass in order to make the AI even want to sue for peace.
I think if you were to remove this then in multiplayer people could just ignore your demands for example "nah you have taken half my planets but im going to make you capture every single one till i let you get the 1-3 planets you want.".
I already managed to turn worse situations around. Early game War against advanced AI Empire. Granted, the AI I fought was using a full missile fleet, so I only needed to prolong that early war until I had point defense. After that the enemy lost their fleet and got liberated.
Then again, I can take more than 1-3 planets. Closer to 5-7.
But still, many situations can be salvaged. The influence drain is a stupid mechanism. The reason is, that I can drain another player's leader pool, colonization ability and other things, by spamming white peace demands. I could potentially do this as the aggressor even!
Maybe in the future you lose unity, or the People eventually tire of you and your ruler is deposed via coup d'etat, but this mechanic is too exploitable.
It works in EU4, because diplomacy in that game is actually engaging and interesting. (And you don't need to pay unity for leaders and expansion.)
Meanwhile, I can't even send mercenaries, or lend lease equipment to my friends to support them indirectly.
Nor can I trade planets with other empires.
The warscore system should be more like CK2, in that the majority of warscore is gained through the actual war targets.
Also, I can't imagine that you would just let a another player run off with your homeworld. I'd rather fight until all is said and done, than give out my first planet.
Also, am I the only one who would like to see, that you can only annex a planet that is occupied? It makes sense, if you think about it. Why should the People of the planet "boule de glace" allow another empire to take over, when they were not involved in any fight?
Liberation is different, since you just tell them they gain self determination, but even then, if the ethics of the people are too drastically different from the puppet government you set up, I'd like to see them rise up and depose of the foreign puppet. Maybe they should even try to reunite with their parent empire.