There is something to be said about the laptop form factor and macOS enabling productivity. That said, I would definitely see the guts of an iPad being put in the frame of a laptop with macOS.

MacOS generally needs more ram than iPadOS because iPadOS can be more aggressive about pausing and closing applications to free up more ram than MacOS can be. I think some modern iPads are still at 4GB when that's too low for modern MacOS.

The Air and 2 port Pro are very close to being the guts of an iPad Pro in a laptop chassis with macOS anyway.

Different input devices really do need different UI controls, if you want a comfortable and polished experience. You can just do it anyway and expect the user to 'deal with it', but you end up with experiences that people just don't really like: https://en.wikipedia.org/wiki/Microsoft_Tablet_PC

an iPad with the Apple keyboard brings input parity, so really nothing holding back running Mac apps.

An ipad with a keyboard would still have an issue with the control design disparity between touch input vs pointer input.