As I noted in another comment, it was a remarkable accomplishment that Mac OS X was reasonably recognizable to Mac users and ran the same apps (via Classic and

As I noted in another comment, it was a remarkable accomplishment that Mac OS X was reasonably recognizable to Mac users and ran the same apps (via Classic and Carbon), all while having a completely different core OS under the hood (though as another poster noted they had prior experience building Mac-on-Unix environments.) Apple has pulled off a number of surprisingly successful "brain transplant" transitions for the Mac platform, including moving from 68K to PowerPC to intel to ARM. In each case, the user experience remained largely the same and old apps continued to run via emulation.