It's not hard, and would have been done ages ago, IF, you don't mind losing compatibility with many (most) prior applications.
Copland tried to do it all. Provide all the modern features, maintain backward compatibility, and operate at acceptable speeds without compatibility environments, or dual booting. Further, there were many new features they were tring to implement. It was too much at once.
This was an especially hard task when you consider that some applications directly patched (altered) the OS, and/or directly addressed data structures and locations beneath the API.
I'm sure it was known that some apps that would'nt make the transition. Still, look at how much time was necessary to release OSX. This was (in part) due to carbon. A strait port of BSD or NeXT Step would have been fairly quick.
Lest I make the wrong impression, I singled out Carbon because it was not part of the original plan.
Memory protection, PMT, SMP, and the rest could have been done, perhaps, more quickly than creating OS X. However, it would not have had the features of OS X, nor would it have gained the mind share and abilities added by it's Unix heritage.
I can't judge, but today's classic may have better compatibility than Copland would have with traditional Mac Applications.