I personally liked iOS and macOS being separate things because making a desktop OS also work on a touchscreen has wider implications than it sounds. That's why these days everything in Windows is blown up like Fisher Price software and way bigger than necessary for a mouse cursor. Seems like that's the direction Apple is headed in anyway with Tahoe.
The phone in my hand is powerful enough to handle all the general purpose computing I already do, so let me do it Apple!
The actual hardware system differences between an M4 iPad Air and M4 MacBook Air are pretty slim as far as the OS would be concerned.
You can connect an iPad to an external display, keyboard, and mouse. It even has multi-window support.
Not supporting Mac apps on iPad OS is a product decision by Apple, not a hardware or underlying OS issue.