Mobile devices will presumably jack in to large monitors or head mounted displays of some kind, plus a good keyboard of some kind, and pointing device(track pad). Thanks to Moores law you should be able to have the equivalent of a desktop experience driven by a mobile device soon.
The one problem is being slave to Apple and Google having absolute control of the OS, software and apps you can run. The only plus is their hegemony does help with security but neither one of them is even remotely trustable with the keys to every personal computer on the planet. There is irony that the creator of the big brother commercial in 1984 is increasingly, you know, big brother.
P.S.
Not sure how Daves self promotion blogging rose so far on hacker news. It actually has almost nothing to do with Doug Engelbart. He was just name dropping, something he often does, before doing what he usually does, self promotion. Yes there is that age discrimination angle which plays well with some old programmers, but not me.
People who spend their time complaining about discrimination should spend more time doing something that people will value regardless of your age, sex or gender. At that point your age, sex and gender become irrelevent especially on the Internet which tends to be blind to these things until you wear them on your sleeve. Dave likes to use his age to promote a martyr complex angle.
Ive looked at most of Daves recent projects and none of them are particularly interesting.
Do you happen to have a TV? A fine large display for most folks.
For pros (programmers) there will continue to exist dedicated large displays, pro keyboards and whatever else. Look at professional gear of audio or video engineers, or CAD users, they have pretty impressive devices to use with computers. It's just not what mass users have at home.
Fully offline desktop computing will remain possible, much like running a C64 or an MSX machine (possibly in a form of new, much cooler hardware) remains possible. It's just not very relevant.
They don't, but they're actually not bad analogues for the computing experience we had a couple decades ago. The original 128K Mac (https://en.wikipedia.org/wiki/Macintosh_128K) had a 9" display, after all -- not that much smaller than the display you get on an iPad Air. I bet someone who learned how to develop great UIs on those Macs would be right at home doing the same on a smartphone or tablet.
How would be that possible?
Good spirit and even better understanding of the Zeitgeist :)