Turn that around. Thought experiment. Say we work like pigdogs for 5 years and we completely replace all of Latex except the part that produces pdf. What value have we provided to users at that point?
What is generally bad about the PDFs that Latex produces (and is a problem with latex, not a problem with PDF) is that they are very inaccessible, they don't work with screen readers.
The reason it's so hard to make latex output HTML (although people are working on it) is that latex is actually a programming language, which is executed to decide where things go on in the PDF.
Make latex output HTML is a bit like trying to take (say) a game engine like Unity, and change it's rendering engine to output HTML instead of graphics -- in the worst case it's basically impossible, as the game just generates commands like "draw triangle here", without context or semantics.
Note how Richard's book adapts to any screen size, can change fonts and color schemes, system settings such as 'high contrast' will affect the rendering of the page, and you could even use browser extensions to restyle the page to e.g. use a more dyslexic friendly font of your choice.
This kind of functionality is not afforded by Adobe Reader. Even the official Adobe's example of reflowing that was posted in another thread is quite bad: https://helpx.adobe.com/uk/acrobat/using/reading-pdfs-reflow... The reflowed PDF is just stacking all text and removing all non-text visual cues. For example, pairs of name/role are separated by whitespace in the PDF, but after reflowing they are undisguishable from each other (who would be the senior VP, Sunny or Daniel?). In HTML, reflowing would preserve semantically relevant whitespace out of the box.