The reason there's not more SoC desktop/laptop solutions comes down to how buyers have been trained to think about their purchases. And the thing they've been trained to value is "upgrading".

People think they want a computer that they can change the GPU, CPU and memory in as their budget allows and their performance needs dictate. Since this was how every computer since the original IBM PC was delivered, this training is now generations old and deeply embedded.

But you know what? 99% of every desktop is never upgraded. They're used in the configuration they are delivered in, until they're "obsolete" then they're either handed down, or discarded. Outside of a small group of enthusiasts, and a small group of IT pros, once the case is screwed shut, it stays shut forever.

Apple is gambling that their customers will abandon the perceived value of "upgradability" in favor of performance right now, and that they'll treat their computers they way they have been trained to treat their phones: a device you use for a few years, then replace to get more power and capability. (this is what they ACTUALLY do anyway; all Apple is really gambling on is that the customers won't revolt at losing the POTENTIAL to upgrade).

The one place I think they do run a risk is with RAM. If you guess wrong about the amount of RAM you need, you can never fix that with an SoC. And RAM is (relatively) cheap; having the ability to add more does give users a viable (and relatively simple) option to get more work done on their computers without requiring them to join the ranks of the enthusiasts or the IT pros.