The likely one to suffer will probably be GOG not Steam. They survive on thin margins and the Epic competition, such as it is, will make it tougher for them. Luckily they have Cyberpunk 2077 coming up sometime in the future. For me this is annoying because I like the DRM free games and extra compatibility they offer.
Its like replacing a jungle with a planted forest. There is no undergrowth and the overall ecosystem derives little benefit from it.
Unless someone else starts offering DRM-free games, I don't see GOG going anywhere. There will always be a market for games that don't require a client.
I think Steam has been hurting GOG for a long time though. Remember when GOG was the only place to acquire digital versions of classic games? That was originally their specialty. It was the go-to site when you wanted to relive those old favorites on your modern system.
Then Valve decided to jump on that bandwagon, and now almost all of those classics are available on Steam as well. Now the only thing GOG has going for it is being DRM-free. Hopefully that's enough.