The iPad does this (it surprises people who see you extending your iPad desktop to a 4K monitor, and using your Magic Keyboard w/ touchpad to drive them) and it's not hurting Mac sales any. The premise that Apple doesn't allow this for that reason doesn't make sense.
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[ 4.1 ms ] story [ 18.8 ms ] threadPeople are using iPad OS features on their iPhones
https://news.ycombinator.com/item?id=45950408