Why can't Apple just sell native macOS support for iPad at a rate where they wouldn't cannibalize their other offerings? I'd pay around $300-$400 for it without hesitation.
It's not about the money, it's about the principle. The entire reason why the iPad exists[0] is because they think fingers touching mouse software is a bad idea. Some people couch this request in terms like "well what if they made macOS touch friendly", but the answer to that is "that already exists, it's called iPadOS."
What you actually want is for iPadOS to shed the limitations of the mobile OS it evolved from. That's a whole different set of asks; many of which cross different but equally strongly-held red lines. A lot of the features of macOS that make it useful for developers - the native UNIX shell, Virtualization.framework, third-party distribution, the ability to relax signature verification on software[1], files that live outside of app containers[2], and most importantly, root access - are all things that Apple considers outmoded and insecure. Insamuch as macOS still supports them, it's because software developers require them to work, so Apple has a policy of keeping software development corralled to macOS instead of letting developers and their attendant security issues spill over into their "device platforms".
[0] Going all the way back to Steve Jobs having his engineers make a tablet computer demo out of sheer spite for Windows XP tablet edition
[1] Or, on Apple Silicon, outright just sign your own OS kernel
[2] On other Apple platforms, your "On My iPad/iPhone" files live inside of a special container for the Files app; and there's another container for iCloud files. There is technically still a home directory, where all your app containers live, but you can't see or interact with it unless you jailbreak.
I'm not sure if they would canibalise anything much. The sell each year around:
1) 230m iPhones
2) 50m iPads
3) 25m Macbooks
Macbook is pretty much niche product to them comparing to Windows market share. Many would still wanna own Macbook even if iPad would support MacOS. They would sell much more iPads and bring bring more users to their ecosystem, familiarise those that used Windows before and maybe they would buy Macbook later on.
