You're thinking in terms of consoles. There are tons of games that only release on Steam, people would just buy those. On other games that are also on Epic and GOG, it's still something like 85-90% of sales are through Steam. If you don't game on a PC, you won't understand. Steam isn't just a store, it's an entire platform. And all of its competitors attempt to compete by just being a store. And that's why they lose. Epic just recently added the ability to gift games to other people, it's been out for 7 years, and just now lets you do gifting. It took something like 5 years to get a cart.
Steam has had that for over 15 years. It also doesn't just have a basic contact list and some text chat. You can do voice and video calls through it while games are running. Invite people to play games on Steam, or watch a stream of your playing, or on supported games see the stream, but also join in and play a couch co-op only game entirely through a single copy of the game. It also has no brainer features like selecting friends and it will filter the list of games you own down to online multiple games everyone owns.
Every single game that has its own message board, and a user content section to let people upload guides, screenshots, videos, and reviews. Games can also support Steam Workshop that lets people upload mods for games which others can search through and install by clicking a single install button.
The Steam Machine will be just like the Steam Deck. It will come preinstalled with Steam OS, but it's a PC, install Windows on it if you want. Or even if you want, from within Steam OS you can install other launchers like Epic, GOG, etc and buy games there rather than Steam. Or you could even pirate games if that's your thing. Some people will of course do that, sometimes it's the only option for some delisted games locked in licensing hell. But the vast majority will just buy games on Steam as it offers a better experience. And if you are going to put all your games into a single store would you rather it be because you are locked in? Or because you get a superior experience?