That's on developers for not putting their games on other platforms, Valve do not prevent you from doing so. If they went crazy tomorrow, people can just jump ship.
I swear the only games that could never be on another store would be Valve's own. It's really not their fault that other platforms are so bad or niche.
Like realistically what should they do to not be seen as gatekeepers? Become worse to scare developers and customers onto other platforms?
I'm with you on all of this. I'm familiar with this (am a game dev) and you're 100% right that the biggest cost is game distribution. One thing though: it costs ~$100 to list a game on Steam, which is returned to you after it's made a thousand or two.
Honestly there's nothing much valve can do to appease people, but I believe the most likely thing they can do is release data on how much distribution costs and give companies the ability to disable the "extra stuff" to save even a few percent of their revenue.