Dex would be similar to what Apple does with the iPad and a monitor, yes. Apple notably doesn't even do that much with their phones though.
To be clear on Apple's strategy: it's to run all new software platforms through the app store exclusively regardless of whether they could provide a better product outside of it. It's all about the passive income of taxing all app transactions. If they could get away with closing up MacOS, they'd do it in a heartbeat. Fortunately they can't.
The iPad is 15 years old now, and Apple's own iPadOS apps still haven't reached feature parity with their MacOS counterparts. MacOS can run iPad and iOS apps natively. There's no reason Apple couldn't move at least the pro versions of these devices to a MacOS base with the springboard launcher remaining the default UI when used as a mobile device. No reason, other than milking users and devs for additional profit of course.
I came into the Apple ecosystem when it was a scrappy underdog and a far more open platform than now. Intel Macs had bootcamp and official windows drivers supplied by Apple. There are ARM versions of linux and windows, yet Apple used the ARM transition to drop that feature. iMacs used to have target display mode so that they didn't become ewaste at the end of their computing life, but that's gone. Time-machine still exists, but was never ported to iOS because it would compete with icloud. I thought target disk mode was gone, but happy to be wrong on that, it lives on in Mac sharing mode, but again is a mac only feature. The corporatization of Apple has had many consumer friendly casualties and even more paths deliberately never travelled.