I'm not sure I see the problem here. If some apps are only available through shitty ad-packed vendor-controlled stores, then hopefully that'll push people to simply avoid them. I mean, if I want to video-chat with someone and to use Skype/Teams I have to download the Microsoft store app and suffer with all that, or I just could use Facebook Messenger on the regular Google Play store (assuming they don't force their own store like MS) and it's easy, I'm going to tell my friend, "let's use FB Messenger instead; these MS apps are a pain in the ass." And if someone insists on using some shitty MS app that I can only get through the shitty ad-laden MS Store, I might re-evaluate how much I really want to chat with them. Smarter app vendors are going to try to avoid putting their users through that experience.
(Also, this is just an example; for all I know, Meta/FB in this possible future would be the stupid one pushing an ad-laden store while MS might be the smarter one making it easier for users to install their apps.)