I don't agree, mobile and desktop apps are not the same thing... So it would not nullify anything. The 30% cut is not the main reason stopping desktop software houses from porting their apps to mobile, the main reason is they can't be bothered to redesign their apps to work with touch for mobile devices.No, the law would enable apps to be put onto the iPad outside of an App Store, removing any gains for Apple and therefore nullifying this insistence that the reason Apple don't put macOS on the iPad is due to missing out on AppStore fees.
MacOS as an app (a la Sidecar but much more robust) is a great idea, but I think that a Mac will (and should) remain a necessity for this type of feature. I would rather they went down the route of a cheaper Mac Mini style (appleTV/Raspberry Pi sized) dock type thing than force macOS on tablet users, with all of the compromises/complexity it brings.
I have mentioned the idea of a dock many times on MR, and the best for factor in my optionin would be a (desktop) magic keyboard with a M-series chip and some kind of tablet stand (à la Surface Book)
But I still think it wouldn't change much, it would probably be too heavy (it would need a battery) and not seamless enough like the Magic Keyboard.
The main reason people want MacOS as an option on iPad (again noone is forcing anyone contratry to what you said), other that all the el cheapos that want to spend less money, is to be able to travel light and have cellular on the go with one device that can be both an iPad and a MacBook. Most would probably have another Mac or Windows device at home anyway... But if you add too much bulk or if the device is complicated to use or needs to be plugged it, it just defeats the purpose....