I think that that software industry philosophy now applies to many other things. I guess because most things now run software, from cars to farmers milk tanks. But it's probably not the only reason.

> Steam itself is not necessary and is just another system to lure users and developers into a Vendor Lock-In

This ignores the history of how Steam came about in the first place. The PC gaming industry was basically dying because physical distribution was expensive, which made the games expensive too, so a lot of users just pirated games (by downloading them from the internet) because it was easier. Valve recognized that piracy was a symptom of a service problem and set out to create the convenient service experience of piracy in legitimate form. It was actually widely hated at first because people assumed malevolence, but over time Valve has proved their good intentions with the platform. They helped kick off the indy explosion with the Potato Sack, they allow developers to sell Steam keys on other stores without giving Valve a cut, they never ask for exclusivity, and despite what many internet whiners complain about they actually try pretty hard to have worthwhile recommendation and review systems.

> Everything useful it provides should be part of libraries. In a better world - we would use a package manager or Flatpak to install games.

This is basically already the case. Steam is the manager, and it also provides the libraries for its functionality (DRM, workshop, achievements, etc) that are entirely optional. There are games on Steam that don't use any of that and you can copy them to a system without Steam and they run fine (BallisticNG was like this last I checked). Hell, as EA and Ubisoft proved you can even install and run your own alternative client as part of your game you sell on Steam (though thankfully the store now warns users about this behavior).

I agree with you on everything except for Steam. It is the only launcher / store I have installed because it legitimately adds value to the games I play. Specifically Big Picture and In Home Streaming let me play my games on any other device I want, and that's done a lot of mileage for me in playing games like Final Fantasy XIV on the Nintendo Switch, or even on my old laptop sitting in bed.