Sovereigns surrender if their faction power is a lot less than yours.
I made a post before about sky rocketing faction power through city spamming earlier with Master Scouts and using it to force people to surrender. You can actually declare war and demand surrender immediately after in the same conversation.
I beat Expert in less than 50 turns using this, so yeah it probably does need to be fixed.
Yeah, that's almost exactly what happened in this game - I didn't have Master Scouts (playing as Yithril) but got a really strong starting position in some riverlands with four decent city sites and relatively weak monsters, so I soon had a strong economy and lots of troops (who didn't cost too much maintenance thanks to Verga's Warlord ability). Only I've been trying not to abuse surrender so instead I just bullied them for tribute for a long time while exploring the tech trees and building up my cities, until Tarth picked a fight with me and my relations with all the kingdoms took a hit.
I do hope they don't outright remove the functionality, though.
I really like that once you have well and truly broken their backs, you can finish and turn your attention to more important matters.
Oh absolutely, I love it, but just think that it should be subject to the same rules as peace treaties, only more so, so that the AI has to be truly beaten before considering surrender.
At the point I declared war, Markin himself was leading two armies right through my territory towards Magnar, and his military power was about 100 less than mine but still the second best in the game, so he was hardly a spent force.
I'd like to see demand surrender only become an option once the AI has only 1 city left, or something like that. You should at least be forced to fight a few battles before watching the guy keel over.
Yes, that sounds about right. I've made it a self-imposed rule that I have to take the enemy capital before forcing a surrender, just so that it doesn't feel like abuse.