Earlier quoted context omitted.
That strikes me as a lousy comparison. I could easily see a notebook with a detachable touchscreen and the proper OS (OSes?) being a useful machine (easier to see since a reasonably selling device actually exists). What if an iPad Air could simple attach to a MB Air chassis and only serve as the display when attached?
It's not a hardware limit that keeps mobile from wholly eating desktop, but a software one. Mobile OSes are intentionally crippled and locked down at the OS layer. You don't own or control your device, and only approved software can run. Android is a bit better than iOS in this respect, but not much. None of the mobile vendors have any incentive to change this, since it would mean forfeiting the App Store tax and for…
The App Store "tax"? Sure, Apple doesn't mind the cash. But they are first, second and third a hardware company: that's where the real money is. The reason they have no intention to allow side loading apps on iOS has to do with user experience, eliminating support headaches and security (order may be different, but these re the reasons).
The fact is your dream device would appeal to the same people who buy desktop Linux machines now. They exist, but they are a tiny part of the market. Nobody can stay in business catering to just those customers.