Earlier quoted context omitted.
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 even had a DOM renderer which should in theory have fixed approximately all the issues (I don’t know for sure, I never knowingly encountered anything using it), but they’ve killed it off fairly recently, doubling down on the fundamentally unfixable direction that is the pure-canvas approach.
* Inspect the HTML tree containing the ARIA attributes generated by Flutter.
Wouldn't that help with SEO as well?