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? I'd be willing to bet that the answer is in the single digits. And for most of those, that limitation is likely to be entirely arbitrary, instituted by a developer as an anti-consumer form of lock-in.
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.
And because downloadable and cacheable progressive web apps are a thing, as well as local storage options for browsers, the experience for an end-user of a browser-only phone wouldn't need to be any different in low-signal or high-latency situations.
The web is a mature and proven platform for delivering arbitrary code and data, plugins make the web more accessible and easier to use, and web standards make the world more open. It's not a perfect platform, of course, but it's the one we've got; I think making it the default rather than the fallback for the devices most people use more than any other would be a great boon for the world at large.