Probably the most significant rewrite was from the original 68K assembly, not surprisingly championed by Jean-Louis Gassee who also opened up the hardware side.

Once code was in C (which was being standardized), they could work on porting across CPU's. The huge mistakes of Copland and Taligent were largely due to trying to also jump into some variant of C++ and object-oriented language at the OS level. Of course, that was at the same time Linus stuck with C for Linux and made much more relative headway.

This also gave Apple a lot more leverage in the application space. They spun applications out to Claris originally because the apps were such a pain point, but once C and MPW settled they could duck MS O$$ice leverage/fees with their own more integrated approach (and sell Works + round pink blobs to creatives et al). That made it feasible for Steve to ink the deal with Bill to keep Microsoft apps relevant on Mac's, when most of the other vendors were giving up on that 10% of PC share.