It's not crazy to have a device-independent presentational document that also contains enough data to enable some device-dependent rendering of its text and media contents.
I read a boatload of 2 column PDFs on my phone. Liquid Mode has been on PDF Reader on Android for... it feels like a year now? And it's fantastic. Without it I have to constantly pinch and zoom and swipe to the upper right to start a new column, etc.
I hate PDFs, but if I want to read a document I've stored in print fidelity from the 80s on my phone, Liquid is rock solid.
Not OP. Xiaomi MIUI bundles WPS office, which has a "mobile view" for PDFs. It's far from perfect, but it can single-column some multi-column PDFs. Some PDFs, like restaurant PDFs are downright butchered though.
According to the article Adobe reader for Android got this feature recently. I tried it and I am disappointed. Maybe OP was talking about reflow mode that indeed was accessible in many pdf readers, but for technical papers (math) it was useless.
That would be the case if a PDF was solely used for print.
In the digital realm though, a PDF should also be accessible to screenreaders for the blind, people with sight problems (increased fonts, contrast), dyslexia, etc. Those already require alternative representations.
And of course nothing bad about having the best of both worlds! A format that is pixel perfect when you want it too, and adaptable for easier reading when you don't.
It does not sound like the goal is device-dependent rendering, though. This is exactly parallel to Reader Mode, which is a temporary user toggle. The goal of PDF is still to produce documents that mirror print-ready layouts, something that HTML has never really accomplished. Whether you want to consume the documents that way will now be up to you.