Feel like that's a bit comparing apples to oranges. Apps weren't originally acquired through some store on Macs, that's a fairly novel thing. There were package managers and such before that but you'd more or less always get software from the vendor. Disregarding that the original iPhones didn't have apps, as long as apps have been a thing they've always come from the app store.

Sure you can root it and get apps from Cydia and whatnot (if that's still around) but I really don't think many apps will migrate away, at least not fully. Users are lazy, and installing a separate app store or getting an app elsewhere is too much work for some. I don't think you and I fall into that category given the platform we're having this conversation on, but the fediverse is "too unapproachable" for a lot of people, even tech savvy ones, because you can't simply download an app and sign up.

I'm in favour of third party app stores (or just the ability to install apps through the browser, no store attached) simply because I'm miffed my Apple TV cannot run Xbox Game Stream.