Sure but I prefer that to the alternative of ‘everything is only available on the Windows store’, where Steam doesn’t exist

Most games except the ones you listed are available across multiple stores, and websites like humble bundle. Typically the only limitation is that most stores do not allow publishers to advertise that an item is available on another store. For example, Halo can be picked up on Steam or Windows Store these days. Microsoft realised that people preferred certain stores over others. So much so that Epic is still giving away multiple games every week trying to build up it's user base.

EA tried to play hardball with their Origin store and only recently started selling on Steam again.

What happens to my games when the store I bought them from goes bust?

Depends on the store. GoG and Humble Bundle offer DMR free downloads typically so you can store and keep your own software. Others? Same thing as any other walled garden.

Once a program is purchased from a store, it doesn't need to be launched from that store. GOG Galaxy 2 accumulates the storefronts into one launcher, where you can search, install, and launch any game that you've purchased on any of the platforms. This wouldn't even be possible on an iOS device. I'm in favor of openness and interconnectivity over uniformity.

> as a user I don't want 20 goddamn app stores on my device

Nobody would force anyone to install alternate stores.