I think you're a little paranoid. Look at Android - alternative app stores exist, yet virtually everything of note is distributed via the central marketplace that is Google Play. I honestly think your concern is a non-issue because 9 times out of 10, people will use the default store.
You're worried about Meta or Amazon or whoever exclusively distributing their software via their marketplaces, but this simply has not happened.
That is your opinion and you are certainly entitled to it. I disagree and do not believe "paranoia" has anything to do with my opinion detailed here:

Japan Passes Law to Allow Third-Party App Stores on the iPhone
I have said this many times: It hasn't happened YET because TODAY companies could only open an Android store. If alt-stores are forced on Apple world-wide then the likes of Amazon, Epic, Meta, Steam, etc. will gladly open their own Mobile stores and move their own apps to it exclusively and...
It is my opinion that Alt-stores and exclusive distribution agreements have not happened YET. Why open a "mobile" app store when you can only service Android? You wouldn't, you would wait until you can service nearly all mobile customers, Android and iOS. Do you really think that Epic, for example, will sit back and only move their apps to their store? Nope, they will incentivize others to increase traffic to their store or web site, traffic is everything. Meta will do the same for their VR headset apps. Steam will do the same to keep pace, on and on and on.
I would probably bow out of this debate if as part of the developer agreement devs were required to maintain a presence on the Apple store. This way the "one stop shop" aspect of the iOS store would remain intact and I would be able to limit my exposure to fly by night payment processors.
This is nuttier than fruit cake.
How about debating without insults? While I might also consider it "nutty" that you don't think it will happen I choose to not belittle your opinion.
No developer is going to lose revenue by doing this. How many apps in Steam are exclusive to Steam?
Well this is your opinion but first off, Steam currently is servicing PC/Mac which is different than the possible advent of mobile gaming stores.
Secondly:
If what you’re saying is true then why hasn’t it happened on Android? Android has always allowed third party stores yet all of these apps still exist on the official app store.
Keep reading on to post #102 or the summary above, also, it is not "true", it is my opinion of what will happen IF alt-stores are forced upon us.

Japan Passes Law to Allow Third-Party App Stores on the iPhone
I have said this many times: It hasn't happened YET because TODAY companies could only open an Android store. If alt-stores are forced on Apple world-wide then the likes of Amazon, Epic, Meta, Steam, etc. will gladly open their own Mobile stores and move their own apps to it exclusively and...