Why not just require apps to be distributed as LLVM bitcode, as is done for the Apple watch? That would give Apple the flexibility to change architectures--or even to create its own instruction set, now that it's making its own chips.
It's not worth the overhead of attempting to ship bitcode. Apple has shipped products on 6502, 68000, PPC, x86, x86-64, ARM, and ARM64. The transitions have been annoying for the lost software, but that's not an unusual occurrence for an OSX upgrade either.