Indeed, Safari's reader already upgrades to using the rendered page, but even it fails on more esoteric pages using e.g. lazy loaded content (i.e. you haven't scrolled to it yet for it to load); or (god forbid) virtualized scrolling pages, which offloads content out of view.

It's a big web out there, there's even more heinous stuff. Even identifying what the main content is can be a challenge.

And reader mode has the benefit of being ran by the user. Identifying when to run a page-simplifying action on some headlessly loaded URL can be tricky. I imagine it would need to be like: load URL, await load event, scroll to bottom of page, wait for the network to be idle (and possibly for long tasks/animations to finish, too)