But those are still digital-only platforms, with a chance of them disappearing. Epic is the biggest risk there, I think.
GoG is an interesting case though, it has loads of games that by and large were available on physical media, but because said physical media is either gone, broken, or in the hands of collectors, getting a physical copy of those games is difficult now. Them being a digital platform re-enables people to play these games.
GoG is also DRM free, so if GoG dies it's not like you'll lose access to your games. Even if you lose the files, archives will exist. Plus, if you're really that morally opposed to file sharing, you can always put it on a NAS or flash drive. Heck, put it on a bluray if you want to
Plus GOG allow people to download games and store them on a hard drive.
UT99 can't be bought anymore (though I think Epic now allow people to share it freely), but I can still download it (for now), and access my archived copy.