The biggest problem isn't sizing or font sizes, most designs stretch and scale and deal fairly well with that like even if you kick it old-school and use tables.

The big problem is catering to different input methods. A mouse has far greater accuracy than a touch display. You can put two links mere pixels apart on a desktop interface and that is fine because a mouse is more than accurate enough. The smallest interactive element a keyboard-and-mouse user can hit is probably about the size of a single period in a font. There are other issues with doing that, I'm trying to highlight the sort of accuracy you have with a mouse. A mobile user could never hit such a small target.

On PC, you can enter text and show the full content of the website at the same time. You can search in the page with a keypress. You can open multiple web pages at the same time. That is not possible on mobile.

Yes, although clicking links on a page with mere pixels between them is still a PITA for many users. I was reminded of this using the super cheap mouse and acquaintance was using on a dinning table, and getting precision was possible but really frustrating. And they didn't like trackpads.

As more and more countries have aging population I'd expect these kind of accessibility issues to be more prominent. Sometimes I feel like using modes (vim style) could help, with the user getting different tradeoffs when "reading" and "manipulating", if there was an easy enough way to switch between one mode and the other.

And miraculously base html/css already solved that. There’s a reason desktop UI don’t work on small screens and that’s the same reason you don’t do magazine layout on small books. Html is reflowable because that’s the only way it can work across different screen sizes. But designers like to think their canvas sizes is the norm and do layouts that shouldn’t be done.

I'm with you on the size issues, though I don't see html/css as solving it completely.

At the end of the day people want magazine layouts, newspaper splash styles, postcard type areas etc. I think even novel writers/editors have a "best viewed at" size and layout in mind that gives a perfect pace to their story.

Html/css gives the tool to switch layouts and potentially adjust to make the best of the area offered, but it will probably always be a compromise in the eyes of the more opiniated designers, and auto-reflowing content would be more of a necessary evil.

Even in our field we have traces of that with our recommended line length, method length, bracket styles etc.