Noting your use of "many" rather than "all" or even "most".
It very well could be most or all. Even with sideloading and alternative app store options, I think it's unlikely developers will abandon the App Store anytime soon. It's still going to bring in a lot of traffic/exposure and therefore developers will still want to be there, as well as other places. Do you really feel the App Store is so bad that developers won't continue to use it after alternatives are available?
Nothing saves money for a developer like supporting multiple outlets...
It can save money if the other options are cheaper for them and generate acceptable traffic instead of having to rely 100% on the App Store for iOS business.
Why would wealthier people prefer iOS? Perhaps because of the experience? Wealthier people tend to prefer curated experiences, so with that gone now, so why would you expect people to keep spending as much?
There can be a variety of reasons including Apple’s image of being a more "upscale" or "luxury" brand. Another factor is that Android-based phones are available at lower prices which attract lower income users. It's not necessarily about being a "better" or "worse" experience.
Will they be allowed to? I don't mean this rhetorically. Jobs specifically described the App Store as a way to make the app buying experience easier and more secure for buyers and sellers. Making it easier in the ways they did is now illegal. What's the motivation to try again if the government is just going to find another reason to cut them off at the knees?
Yes, Apple will be allowed to make improvements to its App Store just as they do with other products and services. Their motivation here would be to keep or attract customers (developers and users) as they face potential new competition.