Let's say someone decides they want to run a game specifically about a group of halfings who have to leave their idyllic home and go on one or more adventures.
Is this an unreasonable idea for a game? Does it indicate the GM lacks vision or can only run railroads?
Is it reasonable for players to advise the GM they're on board with the game, but want to play a gnomish illusionist or dwarven fighter instead of a halfling?
If players do suggest such characters, is it reasonable for the GM to indicate that they'd really prefer their halfing game have halfling characters?
For myself:
If I pitched a "halflings go on an adventure" game to my players, and they started coming back to me with non-halfing character concepts, I wouldn't fight with them about it. I would start by asking, "Are you actually interested in a game about halfings who have to leave their idyllic home and go on adventures?" If the answer is yes, then I'd suggest that halfling characters are far, far more suitable for the concept. If a single player wanted something different, I'd probably be good with it, but more than that and I'd feel the original concept was being undermined and most likely I'd be a little confused about why they're saying they want a halfling game but don't want to play halflings.
If multiple players simply didn't want to play halflings, then I would not adapt the game to become a non-halfing one -- I'd scrap the whole plan (or save it for another day when I have players interested in engaging with the idea) and come up with an entirely new concept, looking for something everyone is interested in.
Importantly, none of this process involves players vs GMs or one side trying to win while other side loses, it's just a group of people looking for consensus and, if the final consensus is, "this particular game idea isn't going to go ahead at this point," that's perfectly OK. There is absolutely no need for any hurt feelings or demands for capitulation.
That said, if the entire group has agreed to a halfling game, and one player suddenly decides they don't want to play along at all, they may end up having to sit out until the next campaign. Most likely, they won't do this in a petulant way, they'll just say, "You know, this game isn't actually for me, I'm going to bow out for now."