It’s impossible, even in principle, to imagine a world without HTML, isn’t it?
Certainly, there are various ways to convey textual content… but textual information will itself be a means of conveying information and ideas for… oh, let’s just call it “the foreseeable future”.
As a consequence, we’ll not see HTML “superseded” by anything else; it will simply evolve. Certain fundamentals – such as marked-up text – will remain. Why not? Until brain-to-brain connectivity is commonplace, everyone will still need text for writing and reading.
PDF has a similar inevitability. Like text, users don’t often articulate their need for it; they just assume. The role of PDF is to deliver a reliable, precise means of capturing, collating and sharing both content and layout. Economies, and society in general, simply can’t live without this capability.
Take away any one of PDF’s fundamental features and you no longer have a general-purpose electronic document format – you no longer have something that can compete with PDF.
That’s why, like HTML, PDF can never die. It can only evolve.