The web had this before briefly. Back in the 90s and 00s, some people just wanted to use Flash. They treated the web browser like it was just a way to run executables in a window. Nothing worked right. Bookmarks, search engines, command-line tools, user preferences for font size, user scripts, view source, right-click menus, ad blockers, auto-translations, user stylesheets, assistive tools, bookmarklets, deep links……
This is exactly how Flutter (now) works on web - the browser loads a JS/WASM blob, and your app renders everything to a canvas - and you're right, almost everything you mention is problematic. There is a time and a place for that paradigm, but it's niche. The lack of SEO alone is usually enough to stop it in its tracks.
Flutter or somethint similar maybe possibly perhaps might some day conceivably eventually rebuild enough platform that the major gaps and faults and chasms of it might be less terribly ruinous, but users will never have anywhere near as much agency. User scripting will always be at the extent the developer's enable it. Its terrible to think of the web denigrated to being as closed foreign and black box as regular apps are, would set back human conputing's one little tenous bridge of progress back immeasurably.
None the less, theres folks who believe the developer experience trumps all other concerns, and are still full steam ahead on canvas based systems. One of those people is Ian Hixie, former editor of the HTML specification, who authored what is sort of the manifesto of the canvas-based effort, Towards a Modern Stack,
https://news.ycombinator.com/item?id=34612696 https://docs.google.com/document/d/1peUSMsvFGvqD5yKh3GprskLC...
Plz plz no.