This is going to be a growing pain for Steam only gamers I imagine. Hang in there, hope that those games fail (?), adjust to it, and do what you gotta do to cope. If Google Stadia gets timed exclusives, I'll respect it and know that I'll have to wait because of data caps. I would be technically locked out, nothing I can do, but with something like EA no longer putting it's games on Steam, or Epic Games securing exclusives, I am not locked out. This just seem like something that's necessary for other stores to get the ball rolling with Steam getting games on their app, and usually games that can only be accessed from their app.
When I first moved to back to PC gaming my first game was bought from Square Enix (FFXI). It actually took me a long time to download Steam, and it was for a specific non Valve game. I was used to getting games from wherever I needed to. Most developers sold their games from their own site. I got Daiblo 3 from Blizzard with no problem. One of the things people say steam is necessary for is exposure, but with google, twitch, and youtube, the cream will rise no matter what I feel. I never even shop on Steam's store page. Then some say it's about features, and yeah there are a lot of them. However people use features outside of Steam all the time, like Discord, and you can still use Steam for non steam games, I personally have to use Steam for non steam games because I like the Steam Controller and the Steam Input (please some third party make a non store front, dedicated alternative with touch pad support). There's a way to use the controller with Steam closed via a custom driver, but it makes the controller a generic controller.
I don't expect or want other store fronts to hurt or be more popular than Steam, I just want to see other store fronts do well enough that I don't have to be locked to Steam so much. Maybe eventually when Microsoft announce their new Forza or Halo coming to more than Windows store, they won't just say Steam, but list many other stores.