For me, it's the opposite. Whenever i have the choice, i want EPUB, not PDF. The problem with PDFs is that you don't have a device that is the same size as the original page size in most cases.
I'll take a slightly-less-great ePub over a PDF I have to scroll around or use terrible reflow heuristics via some reader any day.
Sometimes one can use a landscape-oriented display to avoid horizontal scrolling, but even if the same word count fits on the screen I seem to be annoyed by the low line count.
Providing large type and huge type PDFs would not entirely solve this problem as sometimes even one with poor eyesight might prefer a smaller font for scanning/skimming. Having to acquire two PDFs and switch between them based on mode of use seems suboptimal.
Fixed paged presentation has significant advantages for familiar reference material; some people seem to have a spatial memory that makes finding specific content by flipping pages faster than trying several search phrases (with the occasional benefit of serendipity).
Poetry often benefits from not reflowing lines and page breaking within a stanza is often more jarring than within a paragraph of prose. Yet a reader might prefer inferior typography over having to use a magnifying glass or carry a very large display.
One might be able to get some of the advantages of paged media for figures and tables by having header (or footer) pop-up links to such content when it is on the same "page" as the displayed text. This is not as low effort as moving one's eyes, but it might be better than inlined presentation on a small (relative to font size) display.
Even with print, there would be times when breaking the text to fit a figure is more disruptive than having the figure on a separate page. There would also be times when all the relevant figures would not fit on the same page as the related text.
Having a separate booklet of illustrations might make going back and forth between text and illustration easier, similar to having a lexicon or commentary open while reading. However, that also introduces position tracking in another book and other inconveniences.
Even when my vision was better, reading academic papers distributed as PDFs (usually 2-column) on a computer screen was less enjoyable than reading similar material in a reflowable format. Academic papers also do not seem to benefit as much from pagination as other writings.
I would have thought that a PDF of a book would normally be made in the size of the physical book, which could be A4, but usually isn't (at least not when I look at my bookshelves).
(Also I feel that by default, non-book PDFs tend to show up in the US "Letter" size, which looks deceptively similar to A4, until you try to print it.)
Also: What did people screw up with HTML in your opinion?
The problem with PDFs is that you need to create multiple layouts to make them look good in print and on a variety of commonly used screen sizes; all those layouts is extra work. HTML, by its very nature, doesn't have this problem, and yet somehow today we still have to design multiple layouts to support print and common screen sizes. And in practice, we usually don't - instead, we design one layout optimized for mobile phones, and ignore how bad lit looks on desktop or in print. "Responsive web design" turned into forcing HTML to behave like a PDF, except using "iPhone" instead of "A4" as the size.
As for responsive HTML, it's the responsibility of the designer to make it work if he/she is worth their salt. Like you say, HTML without CSS is already responsive. If businesses understood that there are a big segment of customers who will always use their computer and never their phone when it's time to make a purchase, perhaps they'd be better at it.
I hate to break it to you but, https://blog.developer.adobe.com/adobe-sensei-makes-responsi...
Edit: Also, are there any advantages with large papers like A4/Letter for physical prints, except that you can fit more on a single page?
You must be from North America. In the rest of the world, it’s always A4. I encounter A4 PDFs fairly often, but don’t know how long it would be since I encountered Letter, but easily years.