If I ware Epic I would put Fortnie in Steam but make everything 30% more expensive in Steam (hopefully Steam let's the developer present users information like X is cheaper on Epic and is not like other companies that censors information they don't like).

With Fortninte on Steam the dudes that don't want to be bothered with 2 launchers can use only one(you still need Epic accounts like in all MMOs).

One of the most popular games is Minecraft and it did not need to be on Steam to get popularity, so it is still possible to host downloads for your game on your servers.