I think Steam is not a good comparison. People go to Steam on PC for expensive applications (video games), and for good customer service. Steam actually presents a good value proposition for maintaining your game library. People refused to go to other stores because other stores were crap.

As I understand, there is a significant amount of trivial but essential apps that could be made, and will never get on the App Store, like a calculator that has no ads and no in-app payments. Or for companies like Epic who have big enough IPs that they don't care about discoverability, because they are already viral.

Yes, exposure is a big factor, but if you don't need the exposure, you don't need it. Xbox Game Pass is not on Steam. Microsoft Flight Sim and DCS World both have their non-steam presences alive and kicking, with DCS simply giving you a much better deal if you buy outside Steam, so the niche community around it mostly does.