My take.
Player agency is paramount. To deny it takes away the element that I consider vital to making the game a fiction-generating game rather than a collaborative fiction project. If you're laying out plot-points and forcing characters to reach them, then you are not doing the work of a game-master. You've made the outline for a novel/short story and are then, like Tom Sawyer getting those kids to whitewash the fence, conning other people into doing part of the work for you.
I have had fun getting together with people I RP with and deciding story outlines for events that were in our characters' personal histories, and then RPing them as flashbacks, but this was not the whole game. Nor was it the disingenuous act of a GM pretending that it's a game where I get to decide what my character does while actually refusing to allow me to have said character do what he wants.
So, timelines. Okay, if somebody showed up on GoB and wrote a character who had killing the king as a goal, I'd tell them, naw, we're really trying not to break the canon timeline. If somebody approved suddenly expressed the desire to kill the king, I would say, huh? Why? Like almost all 'canon' characters, and any who were more than a mentioned name, the king is an NPC who isn't present, and odds of any PC developing a real motivation to bump him off via IC events is pretty slim. I'm not gonna let somebody break the world just 'cause they feel like fucking with the world. But if I, or player GMs (whose plots I oversee and am thus responsible for the consequences of) lay down a line of events that leads a PC's personal story into a situation where said PC is gonna try to kill the king, it's my frickin' responsibility as a game-master to give that PC a fair shake at it. The little axiom goes, "Don't say no, determine difficulty." And by funder, if that player comes up with a really clever plan and rolls well, he can bloody well succeed and then. O my god the game will be on an alternate timeline. Though actually, yes, I wouldn't treat that the same as I do most other PC actions, where the difficulty I determine actually gives the players good odds of success, and the plan has to be, not necessarily clever, just not terribly bad. (Feels challenging, but you're likely to succeed, vs. actually bloody hard to pull off.)