It's absolutely not 30% for big publishers.
It says a lot about how large valves market share is that they can take such a large cut and publishers will still sell there. Plenty have tried to release alternative clients without mixed success
Why then install an alternative client?
Source: I tried to compete with steam once. Tech was better. Nobody cared.
Remote Play is fun on games that support it.
And it runs on osx, linux and windows.
And then theres Broadcasting, chat. Its a heavily featured launcher, and offers way more than some of the alternatives out there.
I had all the same gripes you do, believe me. I face similar issues with Steam. Nonetheless, people don't care. It works well enough.