I'd say it's rather common from what I see. Making new accounts for every launcher/store becomes a pain fast just because of the sheer number of those that exist.

The family sharing thing could very well be to encourage people to remain on the platform, I doubt that it is done just to provide a better experience.

The existance of Proton and the focus on Linux is just a backup plan for them. Sure, the material reality is that it is improving gaming on Linux for everyone, but I would keep that in mind, there is monetary incentive for them to do this, specially since Microsoft can become a walled garden whenever it wants to.

Besides they are not single handedly creating a market for Linux, this ignores all the work that was being done before Valve joined in, and ignores everything else going on too. They are a very big reason why, no doubt, but they aren't the only one. If they were truly serious about it all, much more money could be invested on the Linux side and Valve could be advertising it, but they aren't, the closest you get is the Steam Deck and that isn't even available in most of the world.

The problem is that any time somebody other than Valve makes a game store, the only selling point for it is "you can't play this game if you don't download our store." Nobody ever tries actually making something good.

Even if they did, which I doubt is gonna happen, Valve is a de facto monopoly.

No matter how much money Epic throws at the issue, they still don't really compete with Valve. Other solutions like GOG and Itch.io are also not able to compete, and these last two are actually somewhat good.