Let me answer your question with a question: How many things do you do with your phone that aren’t also able to be accomplished with a website already?

This is kinda begging the question imo. Phones are terrible anti-user devices, so I can't do the things I'd like to do with it that I can't also accomplish on a website. Wasn't that kinda the problem that was initially stated in the OP?

Delivering application-like experiences via the web allows users to make accessibility changes to that experience without the developer needing to support it explicitly. It also allows users to implement plugins that extend and improve their experience, by removing undesirable content or adding functionality that you haven’t provided. And because browsers are built on open standards, there’s no longer any device ecosystem lock-in; I should be able to access all of the websites I want to from any browser on any device. Users could even build their own bespoke applications, without the need to enable a developer mode on their phone or get a certification from a megacorp.

Almost all of this would be equally possible if the phone wasn't just a platform for a browser. I actually think a browser model limits a lot of what you say here, and browsers definitely have ecosystem lock-in problems: what Google says essentially goes these days. The browser isn't the great liberator of phones imo.

I don't hate browsers; a lot of what you said is true and great for users with respect to browsers. I do however think it's a weird way to try to fix the phone ecosystem by replacing a restrictive sandbox with a restrictive sandbox that also ties you to a really terrible development ecosystem.