In reality, the question is moot because of the arm vs x86 problem. On top of that, the two OSes along with their corresponding apps and hardware, are designed with conflicting emphases—one for mobility and simplicity, the other for power and customizability. The hardware and software are made for each other specifically, so it’s not simply a matter of mixing and matching whichever software and hardware one wants.
Hypothetically, if macOS could somehow run on an iPad well, I would first ask which iPad we’re talking about. For my iPad Mini, I wouldn’t see the point of running macOS. For my 12.9”, yes, it would be beneficial to have that option since ipadOS doesn’t always do what I need (again, purely hypothetically). In fact, on the 12.9” size device, I never truly need ipadOS other than Pencil functionality, so if I had to choose a single boot OS, it would be macOS, if it supported the Pencil.
But as it is, I can only use the Pencil on ipadOS, and it works out well enough.