I do tend to think 30% is rather excessive but it's still standard across multiple things.. and I remember this talk
https://www.youtube.com/watch?v=stxVBJem3Rs years ago from an indie guy, just a minor remark around the 27 minute point on how when you factor in the cost of doing everything steam does for you on your own, it suddenly isn't such a terrible cost. I have quibbles with that suggesting an all-or-nothing environment when that isn't the case, but the deal only gets even sweeter if you can take advantage of more of steam's offerings besides just being a place to host and sell your game.
On the consumer side too, steam makes things pretty frictionless. I can't say literally all of my budget on games is through steam, but it's over 95%. I was also happy that their steam deck I got recently, in addition to helping the linux ecosystem in theory, is actually not a piece of junk like the librem phone I finally got delivered this year and so helps it in reality too. (Though in the librem case that was expected even at pre-order time.)