I have a hunch and am thinking (maybe more wishful thinking) that MS is just trying to hype up a last ditch effort in mobile phones.
I just can't see a compelling possibility that would encourage widespread adoption, but more importantly the abandonment of existing devices and ecosystems in favour of MS' offerings. The next big thing isn't going to stop people from using or wanting existing apps, so it would have to offer something rather special to compel developers to support it. A larger screen and decent browser experience will only get the platform so far, too many apps are only available as an app and I think this is the big problem MS will have.
However, without mobile, windows is kind of stagnant, and hasn't got much to move. I can see with things like the surface studio and creators in mind, MS is targeting entrenched PC use cases which aren't going to be displaced any time soon for multiple reasons, but that isn't going to sustain things for a company like MS.
So you don't think, a voice platform of "skills" (Microsoft bot framework under Cortana as a superapp), or a large screen situation (folding tablets or 3 in 1s/continuum), or augmented reality (glasses with projected screens), woulld make existing apps far less important, and considerably less in vogue ?
And all of these, folding screens, continuum, augmented reality and total voice control? (All things MS has in the works).
Existing apps are entirely bound in the form factor and input method. As soon as the FF changes, and the input methods change, those apps are then legacy apps.
Take something as present as Samsungs DeX. Its a great platform, but the largest complaint is that despite being on android, there are literally no apps in android designed to run on a larger screen with multi-tasking. Its an android device, with a basically zero ecosystem for the application. That problem only compounds when voice, folding tablets, augmented reality and so on come in. Until the point where having this great touch ecosystem, designed for small screens, really isn't the primary thing you want at all.
Incidentally tablets are a "complimentary" market for the most part (as are phones...ie they don't tend to replace computers but exist along side them), and a new frontier for MS, in which they are the only non-budget player experiencing growth. With year on year growth, and just 5% windows marketshare, there is a great deal of room to expand there.
Their cloud server products are still experiencing growth. And PCs, being in an upgrade cycle, have just restarted growth last quarter. Not only that but the premium smartphone market will stop growing, and start experiencing negative growth really soon, due to saturation.
It's not as simple as "you must do smartphones". That boom is ready to start winding down, and everyone is looking for the next wave of tech adoption.
Really "the next big thing" is exactly what one should be targeting. Because that's where the financial boom will come from. Hence why amazon, google and apple are all trying their hand at assistants, smart watches, smart homes and more, Samsung and apple hybrid forms - they aren't particularly profitable right now, but every one knows that's where the next wave of profit comes from something else, when smartphones slow and get superceeded.
That's just how the tech world rolls. Apps aren't going to mean squat 30 years from now when we have hard AI, and they'll probably mean a great deal less the moment we get bigger portable screens or full voice ecosystems, neither of which are too far off.