Point being, you can do that already for $2000--it's called a laptop.
I could flip this argument just as easily—if Apple doubles the price of a MacBook Pro, I want it to run iOS. Just silly.
As a pilot in training, I can't mount a MacBook on my lap, or in a Cessna 172. the detachable keyboard on the iPad makes it ideal. OK, I can.. but there will be no room for passengers.
As for turning the argument around, you're right, however, iOS is designed for fingers to touch since the iPhone 1, and MacOS has been designed for mouse usage since day 1. The breadth of macOS applications makes it a place to go to get more powerful apps that have already been purchased, and since there is not touchscreen for a Mac, how does someone aim those birds on iOS Angry Birds on a Mac with no touch screen? Those same birds can be aimed on an iPad with macOS Angry birds by treating the screen as a touchpad.
The one thing that bugs the snot out of me on the iPad is the full screen and half screen use of the applications. It goes against how I work. At work, I don't clear off my desk, work on something, then, when the phone rings, clear off my desk, answer the phone, clear off the desk to take notes on the phone call, then set everything back how it was to continue my work. It would be helpful to shrink the window (a la 1984...) pull up a note taking app, take the notes while seeing what I had out, so I could quickly answer the questions from the caller, then minimize those notes and get back to my work. On my iPad, the simple act of copying and pasting makes me feel like I am in kindergarten using those fat pencils.
Using iOS on a mac is kind of mimicked in Control Center, however, there is a translation between the trackpad and the icons. One does not simply push the icon for the thing you want to run, like the iOS devices, but you have to maneuver over to them. If the iMac/MacBook had a touchscreen, then the iOS argument makes sense.
So, yes, having mouse support and macOS support makes sense on an iPad, but not on an iMac.
I do write this as a learning exercise, not as a "you're wrong! do it my way!" type creation.