A lot of my games have been with D&D groups, where it would be hard to walk out from a particular game without walking out from a group. In one of those groups, I recall an instance where the majority of the players were filling seats until the current DM got done with his campaign and a fun DM started running again. So, socially, it's very hard to for players to walk from these groups.
If that was the case, I have no sympathy for the players. They should get a life and find something else to do until the next campaign started or start a game they would enjoy.
On the other hand, I haven't run into many groups that had only one person who was willing to GM, which may impact my willingness to accept ultimatums.
I have never been in a group with only one person willing to run, but I have excused myself from a campaign on two or three occasions. One of those times led to a change, because the GM was not enjoying the game as he was catering to the powergaming hack n' slasher to prevent the guy from whining all of the time. When I spoke up, he was relieved and found out that the other players were going along just play (the game was not bad enough for them to quit, but it could have been a lot more fun if the GM was running the group's normal style before he brought in the one particular player).
I am skeptical of any game run by a DM who doesn't want to run the game unless it's exactly the list of restrictions given before the start of the game. If your world has to be just so, what are you going to do if we blow the hell out of it or walk off the edge?
I have had players, unintentionally, blow up the world and it is great. Then again, once the game begins I am about playing to find out happens and follow the lead of the players unless they are being evil (that is against the no evil character policy) or trying to do something outside the parameters established for the setting (e.g., trying to get to the Forgotten Realms, the Far Realms, etc. which do not exist in my campaign).
And who really wants to run a game for a player who doesn't really want to play that game?
Nobody. If nobody wants to play, they should be honest. Maybe, it means someone else needs to run. However, it is just one or two players not interested, maybe they should let everyone else play and they find something else to do for this campaign.
If one race out of many in the world matters that much to the DM, why can't the one race that the player is playing matter to a player?
It can matter to the player. That, however, means that player should be allowed to play it in the campaign.
I don't feel really sympathetic to a DM running in a stock D&D world who narrowly restricts what characters can be played.
And many DMs here don't feel sympathy for players that cannot accept that a given campaign is not for them and move on to find another group.