It depends on what part of hashing things out we are at. Sometimes I'll say "I'm running a game in x (eberron/raveloft/etc) and the session zero for characters is going to be about characters that must fit the setting along with the rest of the group. Other times I have a group who I feel is better able to handle a more collaborative session zero that includes deciding the world. Even in that second group I've seen players try to force a background while we are deciding the world (often by describing their family keep and such then getting upset with the gm or group saying hell no when they want to declare moria their family thing because that was the time to be inserting high level* world elements) in the first type I frequently run into authors who declare they are being denied and having their creativity shut down when I explain how a backstory conflicts with the setting & ways that can be fixed.. Ironically communication is easier than ever with discord Facebook & everything else but the level of resistance is just as if not higher.
*10,000ft broad strokes overview type not cr/character level.
So I think I'm seeing the disconnect.
When I proposed Thorin, my proposal is pretty much so that I wrote: a one or two sentence outline. Now, I was not aware you run a specific setting or AP (if I was, I would adjust to fit that, but in lack of knowledge, I assume a standard general setting).
Further, I'm not married to the Thorin concept even at proposal, it's generally my first choice and my preferred one in this hypothetical example, but I can be convinced to move on IF my next option is equally cool. (If my idea is an exiled noble who seeks glory and a return of his homeland, I might be convinced to shift it to a priest who is interested in reclaiming a lost temple of his faith beset by demons, but I'm not interested in "you're a farmer who discovered goblins carry gold and opted to try his luck in the local dungeon").
Additionally, just because Thorin has this long term goal, it isn't at the exclusion of all other goals: he can be seeking allies, experience and items of power and then at some point in the future, decide to assault the "dragon" in the keep and complete his goal.
Lastly, my goal is that, a goal. The DM is free to design around and flesh out the idea. Perhaps the dragon was killed by adventurers in the meantime and a different worse baddie moved in. Perhaps Thorin's memories painted events different than what they were (although, I'd still like something resembling my goal: to spend sessions building up to a major event in my PCs life only to have another group of dwarves kill the dragon and establish a peaceful kingdom off camera that we just walk into is anticlimactic to the point of trolling).
This, of course, is all permeated on the idea I get a choice in this at all. When people responded to this, only saw one poster say, "your idea is cool, but what if we adjusted this..." And two responses that said, "hmmm... Probably not. And if so, you're going to jump through a lot of hoops before I consider it. But mostly no." Which isn't a great negotiating point.
Then again, I prefer "yes, and" or rarely "no, but" as styles of gaming. So hard a "no" really disconnects me. If Thorin doesn't work, propose a counter offer. Which brings me back to random generation. Random chargen is a hard no. It says "you weren't lucky enough to play what you wanted" and worse, it allows the DM to block things without the negotiation (I'd have let you play your paladin, but the dice didn't. Not MY fault...)
I know there are terrible players who demand primadonna attention. There are terrible DMs too that use their authority to power-trip on their players. No set of rules, be them empowering to DMs or PCs, can fix bad players.