The desktop PC became the dominant computing platform almost 40 years ago with the introduction of the first generation of affordable home computers (Apple II being the primary example). Even though laptop sales have overtook desktop sales around 10 years ago, the preferred form factor (15"+ screen) and intended use (sits at a desk 90%+ of the time) of the vast majority of laptop owners means the majority of laptops are nothing but desktops that happen to be easier to move to another desk.
Even though the amount of time spent using desktops and laptops in a desktop-like manner is decreasing, it's still a form factor found in every home. There may be more smartphones in that home but in my opinion, any product which is universally present in people's home is dominant even if it isn't being used as often as other options. Another example of this would be the microwave versus the oven. I'm not sure which one is used more often than the other but both are still dominant in terms of their universal presence in every home.
"Dominance" of the PC was more late 1980s than late 1970s, and that's if you count only office use. PCs didn't hit 50% residential penetration until ~1998:
Yes, you could buy a PC in the late 1970s. But they were still pretty rare, and even a freestanding home PC didn't become hugely common until the 1990s -- networking was the missing step.
Mainframes have had buffers put between them and end-users, but are still highly utilised today. And the computerisation of the office was proceeding at mixed rates through the 1980s and 1990s (though well advanced by the early 1990s).
Well, imagine being able to project your "laptop" where ever you want using AR? Then you could use it as your home desktop-laptop and carry it everywhere with you.
Tablets are cool and you can carry them everywhere, but you can't really work important stuff on them since they are still too small.
On the other hand, even if laptops had very good battery life (which usually is not the case), they are still too big and heavy to carry around unless really needed (ie. for a meeting or something like that).
But an augmented reality laptop that lives in your phone is something people might want to use. Not for gaming, but for normal office work, be it programming or using spreadsheets.
And, at that point, regular users might think about obsoleting desktop PCs and laptops.
A projected AR display would only be useful for creative and business work if it can completely occlude the background. I won't be able to see the spreadsheet clearly if light leaks through it through from the window in front of me. Due to limits imposed by optics, complete background light blockage is always going to require a bulky, awkward head-mounted display. No one has proposed any way to get around this issue even in theory.
Well, you can always project the virtual laptop's screen on a pizza box or a wall or something like that. But I agree in general. I'd prefer a foldable laptop, but I doubt the technology for something like that is anywhere near, while this AR laptop could be made tomorrow. There already are virtual machines made for mobile phones, all that is left is connecting them to AR glasses and voila.
Walls and pizza boxes aren't sufficiently smooth or reflective to use as projection screens for real work. An AR laptop couldn't be made tomorrow because the glasses exist only in limited prototype form with poor display quality.
Even though the amount of time spent using desktops and laptops in a desktop-like manner is decreasing, it's still a form factor found in every home. There may be more smartphones in that home but in my opinion, any product which is universally present in people's home is dominant even if it isn't being used as often as other options. Another example of this would be the microwave versus the oven. I'm not sure which one is used more often than the other but both are still dominant in terms of their universal presence in every home.