They require (I think -- maybe just strongly encourage) submissions to the iOS App Store to include LLVM bitcode. It’s easy to imagine them doing the same for the Mac. That would be a pretty clear signal that they plan to switch architectures.
Or they could slowly make it required.
No. You do not submit source code.
> They require (I think -- maybe just strongly encourage) submissions to the iOS App Store to include LLVM bitcode.
Apple does not currently require LLVM bitcode for iOS. It does require it for Apple TV and Apple Watch.
Additionally, LLVM bitcode is not architecture agnostic. It would not solve the problem of going from x86 to ARM.
> (anyone remember fat binaries i.e. the ones including both PPC and Intel executables in one .app container?)
This was also used in Apple's transition from 32-bit to 64-bit on Mac. And later again on iOS.
So yes, if Apple were to transition CPU architectures again, fat binaries would be the likely approach.