The community would face challenges. Like MacOS does not have touch controls etc. Might not be to hard of you can bring the code over from iOS components and do new builds; while being out of reach for "the community" which is more or less restricted to mixing binaries from different sources and applying the odd patch.
Apple's bigger problem would be getting any sort of cohesive environment for users that isn't a confusing mess. Do you have to swtich to iOS to run an 'app' or can you do in MacOS in a window? If you change iOS settings, grant permissions etc, does that impact MacOS for that app or not, what about the other way. If you really make it toggle switch where are my files/data? Can access the respective volumes from each platform or are they on the same volume. The iOS UI does have the files app but most apps don't have filesystem hierarchy concept for navigating. Am I expected to copy documents between them? If so how do I know which is the most recent version. What should MacOS do if I don't have an "application" associated but i do have an iOS app that has the right url handlers, suggest I visit the Mac App store, or launch the iOS app?
When WINE first appeared on the scene people wrote stuff like, just boot Windows if you want Windows. Wine took off because everyone who has ever done it knows multi-boot sucks! It can be alright for really dedicated tasks, like gaming "I don't want any interruptions no mail, no ims, I am killing zombies for the next two hours" but it absolutely blows if it means shutting down all my apps waiting for windows to start, using Quicken for 15min, than booting back to my linux desktop. Yeah a lot of us lived like that 25 years ago - never again.
Apple knows this is a terrible experience and they also know delivering it as a product would just leave a bad taste in the mouth of users and lead to a lot of bad reviews lying around for people to read and be turned off by. Asus and some others have tried this kind of thing with Android and Windows and there is a good reason you don't see those products now. https://en.wikipedia.org/wiki/...