Look at devices with foldable screens and external keyboards like the Asus Zenbook 17. Without getting lost in arguing semantics of if an iPad (mini/pro/regular) sized tablet counts as a mobile telephone, it's clearly not a desktop, and it's easy enough to imagine that desktop computers will go the way of the mainframe.

Agreed. Honestly if I had a way to comfortably write software on my phone I don't think I'd ever use my laptop ever again. Phones are simply too comfortable.

Unfortunately the vast majority of them are consumer devices: unlike real computers they don't come with the tools used to program them. I can't make a new app on my phone and run it.

To your general question, I've heard a professor lament a student stating that all CAD applications should work on phones.

And why is that such a lamentable idea? Modern phones have the processing power these days. Walking around a factory, building sites or a muddy field with a phone in your hand is a lot easier than walking around with a laptop. Plus modern phones have LiDAR and multiple cameras opening up for all kinds of interesting options. Frankly any CAD platform which doesn’t embrace mobile will probably fall out of favour over the next few years.