iOS/iPadOS is the future. Ever iteration macOS becomes more iOS like.
The compute model is changing. I am now old and enjoy doing computers the "old way." I watch younger people wizzing through apps on an iPad and they are able to do 85% of what I can do on a desktop—and if I'm being honest they can often arrive at the same output faster.
There are tons of exceptions, of course. No terminal, limited filesystem access, no Xcode, applications like Photoshop and Final Cut run with a limited feature set, etc.. The list is actually quite long. But every year the list gets shorter.
Somewhere in a room at the Apple spaceship is a chart that has all three OSes converging under a single OS, probably aptly called OS. I'm sure they keep pushing the date off into the future, but it will happen. The numbers are really getting up there too—macOS 14, iOS 17, etc.. If I had to guess, there will be no iOS 20. We will get AppleOS v1.0 instead.
If people wanted what you described, then Surfaces would be skyrocketing. However it looks like according to MSFT that Surface sales are sinking, which might explain why the previous head has left Microsoft:
https://www.zdnet.com/article/can-microsoft-recover-from-the...
But isn't Apple in somewhat of a unique position given that they're the only company that develops a popular tablet OS and a popular desktop OS? Microsoft has Windows Subsystem for Android, but it looks like it only runs apps from the Amazon Appstore [1].
If there was an Apple device similar to the discontinued Surface Book that ran iPadOS and MacOS, I would be intrigued. Using an iPad as a display for a Mac is cool, but it would be nice to run MacOS locally.
Unlike Windows Subsystem for Linux which is thriving, Google's stranglehold on Android via its Play Store renders any attempt to integrate Windows and Android non-viable.
I preferred the Surface Book from a device ergonomics perspective -- but I finally got sick of trying to do dev work on Windows.
So I paid 2x as much for the same functionality instead. And that's why Apple won't make that form factor.
Until then, it's hard to define the right thing for such a device to do when it's logged in to a second account on macOS and you un-dock it.
You'd also take a pretty big hit in disk use for such a dual-OS device, until they do a ton more work to unify a lot of their software (I'm thinking not just of the OS, but their office suite and other programs).