You are right that with OS X you have no particular say over the features. On the other hand, you do not have to deal with poorly supported hardware, and if something goes wrong there are people whose job is to help you. Most users find those things to be advantages, but many will choose open source software because of the various other freedoms they can get. There are also people who choose open source software because they have unusual requirements that proprietary systems either cannot meet or are too difficult/expensive/onerous to deal with.
Losing users is always a bad thing for the open source ecosystem. Lost users means fewer bug reports, fewer patches, fewer donations, and less support from paid developers working for various corporations (easy to forget but there are people whose job is to maintain open source projects like the Linux kernel). Fewer users means less incentive for hardware companies to provide any support, even just minimal technical documentation, needed for open source developers to write drivers (let alone contribute to that).
Forks require a significant commitment of time and effort, and really should be considered a last resort. The time commitment is much greater for prominent projects that receive a lot of activity like Gnome or Linux. Most users, even those with the necessary technical skills, do not have enough spare time to maintain a fork when things break.
Your dismissive attitude is fine for a obscure hobbyist project with a dozen users. It makes no sense for desktop Linux distros that now have many millions of users.