They had to spend an additional year of development adding the Carbon API so OS-9 apps could be ported. They had to alter the UI. They picked an already established operating system.
You're replying to a post where I said users and developers don't want something 100% new. You said "Apple did it" but they didn't. There was nothing 100% new about OS X. They had to do everything possible to make it as not-new as it could be while being a completely different OS.
Apple moved everyone to a "new to the user" OS. Sorry if that wasn't clear. It's irrelevant that OS-X was based on some other OS. To every user of OS-9 it was a new OS for them. Effectively Apple got all of their users to switch OSes. Microsoft should do the same.
What 10+ year old existing commerical OS should Microsoft move their users to?
You aren't really addressing the point. To users, OS X was the next version of Mac OS. Apple took NeXTStep and added a pile of classic MacOS UI and added APIs so developers could easily port their classic applications. It wasn't 100% new even to the users. Familiar UI. Same apps.
If Apple has just thrown BeOS on their machines and it turned out be a success then there might be some argument for users and developers loving a 100% new OS. But, as it turns out, even an OS as advanced as BeOS is not what users and developers really want. Users want to be able to run their same applications in familiar environment and developers want to make use of their existing code and existing skills.
Another way to look at it is Apple moved their users to their own "Windows NT" -- something that Microsoft also did around the same time frame with Windows XP.
You're replying to a post where I said users and developers don't want something 100% new. You said "Apple did it" but they didn't. There was nothing 100% new about OS X. They had to do everything possible to make it as not-new as it could be while being a completely different OS.
OS X proves my point.