I can sympathize with this, and I'm not married to PDFs myself (they're an aging and not-very-good technology), but I think the key issue is really simple:
WotC have shown complete willingness to shut down digital services people have spent a lot of money on, and other services/websites which probably cost them peanuts to keep up, have likewise been casually shut down.
They refuse to make any promises on the subject, and there's no particular reason to believe that once they're "done" with 5E, that they'll actually ever allow long-term store-able digital copies of 5E stuff to be created by D&D Beyond or Roll 20 or whatever, despite people having paid hundreds of dollars for digital 5E materials. D&D Beyond have said that if they ever shut down, if you use your app and download all the books before they shut down, you'll still have access via that, but obviously if you then had to wipe your phone or whatever, that would be that. You'd have no legal way to get the stuff back (and on Apple, perhaps no way at all without jailbreaking the phone, though I dunno), unless you had a backup and were able to sideload it back on to your phone/tablet, and that actually worked (which it might not, depending on how it was all set up).
I do think it's likely that when 5E is done, WotC will release PDFs of all the books via DMs Guild, but given their pricing on 4E books, you'd likely be looking at spending hundreds of dollars to get stuff you already had.
People want PDFs for a lot of reasons, but I think this is the most common one - the ability to legally and straightforwardly back up their books in an era when that should be trivial.