Most people on desktops / laptops interact with these services via a web browser, which has very limited permissions on the system. Not sure how you could control that tightly on a fully open iOS.

So iOS must allow web apps.

It does, doesn't it? Even if you stretch "web app" to mean "PWA" iOS supports them. And it definitely supports the literal definition of "web app" (i.e. loading a website in a browser that runs JS or whatever to perform its "app" functionality).

According to this discussion, https://news.ycombinator.com/item?id=39563618, they don't behave in the same way or have all features.

E.g., For support, the Progressive Web Apps will still need to be built on WebKit, with all that entails.

I don't see the relevance of this question. Neither what I do nor whether I own a desktop/laptop impact the overwhelming trends of how society interacts with technology.