So is Valve allowing any developer to get its games into their store under reasonable conditions ?

Yes https://steamcommunity.com/games/593110/announcements/detail...

I am glad to hear that.

Yeah, they even allow adult-only games now.

They still blocked few very controversial titles that include rape and some other crazy shit, but otherwise almost anything can be published on Steam.

Humble and Itch.io could make stores too.

So I will have to convince my potential customers to first install the app store that I can get in then they will have to remember to install my app once they complete the set up of the alternative store.

Is this even working on Android or anywhere?

If you are a small indie app, you can stick with the biggest or a popular store. Likes of microsoft and Epic will fight to get there own stores installed or Apple will loosen is grip.

I've used the Amazon, Samsung and Oculus stores on Android. And I think direct APK downloads from Humble a long time ago.

Good for you but are you going to pay me few thousand dollars a month so I can sustain an independent business? Probably not. What I need is access to a lot of people who are willing to pay me a small sum each for the software that I make. Multiple appstores with uneven access is unlikely to help with that and would probably increase my development costs too.

That's not that big a problem for established and big companies but it is something that I am not looking forward to.

Deploying your software to these different stores probably doesn't involve as much friction as you think it does, and if there is a proliferation of stores one could imagine services springing up in the space to facilitate multi-store deployments and to make the process easier on developers. I mean, if it really is such a worry to you then someone will make a product to help with it because it is a business opportunity. TestFlight, after all, was invented because beta-testing Apple's apps used to be (more) awful.