Then one of the previous players decides to GM, and the game goes on, though maybe with a different setting or even game. "Life finds a way."
At which point that GM will be the one with the authority to decide what is in the game. The role of GM has moved, and the authority has moved with it.
The fact that any of the heavy lifting is optional and/or self-inflicted is irrelevant - it's still heavy lifting. And even a game with 0 campaign/adventure prep or world building still requires heavy-lifting - that of actually running the game itself. In my experience, the vast majority of players do not want to run a game, and are happy to concede authority to someone who will.A lot of that heavy lifting is optional, self-inflicted, and built on the backs of what @Ovinomancer talks about with the whole myth constructed around all the hard work of GMing: e.g., campaign/adventure prep, world-building, etc. This is particularly true if you are running something basic like Dungeon World or using no myth roleplaying techniques.
If Player 4 wants to, they can offer to run a campaign. They will then become the GM. And when they do, they will have the authority to restrict character options. Player 4 can declare that no humans are allowed. If the OP's GM (now in the role of a player) doesn't like it, they don't have to play.