Plug it into a usb-c dock connected to a screen/keyboard/mouse and it's a Mac, put it on the little stand with the magic keyboard and it's a MacBook, hold it in your hand and it's an iPad
My dream device
It goes back further than that too. Microsoft bought the Sidekick and squandered their lead by bringing it onto the Microsoft platform.
I'm reminded of this quote: "A Foolish Consistency is the Hobgoblin of Little Minds."
Laptops and tablets are different devices. We should stop trying to merge them. Every time we do, we end up with something that is a worse version of both things. Tablets need a battery. If the display is detachable, then that creates a weight problem for the laptop. The tablet keyboard is worse than any laptop keyboard.
I said over a decade ago I thought Apple was smart to just have a tablet OS as well as OSX. Don't spend 3 years trying to merge those things. It's a waste of time and gains you nothing. Even the tablet and phone OSs are somewhat distinct (but a lot less so than with OSX).
I always thought Eric Schmidt (at the time) did the right thing with ChromeOS and Android too. There were always questions from people seeking faux "consistency" like "why have 2 OSs? Shouldn't we merge them?" Again, phones and laptops are different things. Let each OS evolve and see if one emerges as a "winner". Otherwise, leave them alone.
And then there were the checkboxes which allowed you to extend the OS beyond Chrome OS's initial limitations. Enable Android. Enable Linux. Enable developer mode. But the user (or administrator) could ignore those checkboxes and keep the machine in a fairly locked down state.
Keep imagining a similar abilities on iPads. A seamless transition from desktop mode to touch and back again. With options allowing you to make it into more of a general purpose computer if you want. But the options can be ignored in favor of the walled garden, if that's what the user or school or corporate owners want.
Switching between entirely different and separate "UI personalities" could work.
I don't think the pointer vs touch UI is the main thing that people have in mind when wishing for macOS on the iPad though, instead a "proper" UNIX-style filesystem and shell, and the ability to install any software outside the app store via a package manager and without Apple's nanny reflex getting in the way.
Apple is very clear about their product differentiation and would never make macOS and ipadOS exact copies. Case in point: when they brought cursor support over, they painstakingly engineered it to work differently and snap to objects on the screen. I think if they did OP's suggestion and had a macOS screen show up when an iPad is connected to peripherals it would actually work out well.