It is too early to displace HTML with PDF.

> PDFs used to be inaccessible

My eyes are not very good. I have trouble reading the font in the PDF. I am using Firefox. HTML lets me pick that a font that I can read easily. I cannot do that with PDF.

> PDFs used to be unreadable on small screens, but now you can reflow them.

I am using Firefox. I cannot do that.

Realistically, how many years will I have to wait until Firefox catches up?

Over twenty years ago, I learnt Web authoring by examining the source which had a profound effect on my career. That serendipitous opportunity I had with human-readable sources will be lost to the next generation with PDF - they have to learn the technology deliberately.