What’s stopping indie developers from hosting their downloads on their own site? Sure, offering all the other features Steam does would be hugely expensive, but that’s why consumers prefer it. Those features aren’t necessary though and people would forgo them for a good game. Advertising would be more difficult but they could get popular streamers to play it or do Reddit or twitter posts to get exposure.

Doing all those things themselves are riskier than using Steam but there’s nothing actually stopping them from trying. Steam takes a big cut but a big cut is still preferable to a smaller total amount they would likely get by not using Steam. It’s not Steam’s fault no other platform has gotten their shit together in decades.