There are several reasons, based on personal experience with discord's limitations, why some gamer would switch.
1) Lack of accessibility. It is very hard to include deaf people in voice chats.
2) Lack of advanced voice capabilities. There is no way to setup channels where there talkers that are only heard by a subset of listeners. If you are using discord voice to coordinate large groups of players it can be extremely hard to manage.
3) Lack of Bot developer support. There are significant issues with how Discord has developed their APIs and how they treat 3rs party developers.
4) Censorship. Many gamers are irreverant / bot PC and the centralized nature if Discord pretty much guarantees that Discord will eventually face issues with moderation and censorship that will drive people elsewhere.
If Fosscord can keep (feature complete) client interoperability while adding additional features for self-hosted servers, there is a decent chance that a significant minority would move to Fosscord.
I would expect that if such starts to actually happen, Discord will do everything they can to put legal and technical roadblocks in the way of Fosscord.