This is going to be an unpopular opinion, but I think the beginning of the end was the invention of JavaScript. Pulling down an unknown chunk of code from the internet and running it is malware. Even if browsers successfully sandbox the JS (a promise which they've failed to keep numerous times) it can do all sorts of stuff that doesn't serve me, like mine crypto (theft of resources) or display ads (adware).

The primary benefit of web applications is they don't lose your data. Not a single web application UI that exists provides as good a user experience as the native desktop applications that came before. A web where browsers provided their own UIs for various document types, and those document types could not modify their UIs in any way, period, would be a better web. You serve up the document, I get to control how it looks and behaves.

They do and it's called "reader view".

Browsing without reader view enabled by default is like driving your car around with the hand brake engaged.