You're ignoring a ton of stuff Valve has done. Look at what they've done for Linux gaming at a bare minimum.
If all you're doing is buying games for Windows, Steam doesn't offer much more than GOG. But if you're playing those games, well, that's a different story.
Not to mention that Steam lets you use keys bought from other platforms.
You say that, but I recently had a comment deleted and got banned from a community on that instance. I wasn't being a jerk or anything. The mod was power tripping.