I don't know about that. Apple switched processor architectures twice, and both times software written for the old arch ran on the new one. And when they replaced their entire operating system, they not only made it so you could still program against the old API—just recompile and go—they also made it possible to run the old OS inside the new one so you could still run apps that hadn't yet been recompiled.
And before that, when Apple made the 68k -> PPC transition in the mid-90s, they ran the whole system under a crazy, bizarre emulator that allowed for function-level interworking - 68k code could directly call PowerPC code, and vice versa. Early PowerPC systems were in fact running an OS that was mostly composed of 68k code; it wasn't until 1998 or 1999 (around the release of the iMac) that most of the Toolbox was ported to PowerPC.
