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?
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.