” let me add that if by some cosmic collision Apple decided to throw out all software that had been written for OS X/PPC”
You wouldn’t need to throw anything out. It could all be recompiled for OSX/x86. Mach even supports multi-architecture binaries which NeXTstep made use of when it ran on 68k, x86, SPARC and PA-RISC. I used to compile my NeXTstep apps as a multi-arch binary for all of those platforms into a single app and OSX can do the same. If you take a close look at XCode, you’ll notice that Apple is definitely keeping this option open.