Apple's rumored transition to in-house ARM-based CPUs for their Macbooks couldn't come faster. For such a critical component, relying on a single third party to continue innovating on a timeline which aligns with where you want your products to be is not ideal.
You'd have an emulator, same as the 680x0->PPC and PPC->x86 transitions.
I'm not sure how well this would work in practice. The reasons for the previous changes were that performance of the existing line had reached a dead end, and the new line was noticeably better right from the get-go. Not the case for an Intel->ARM switch.
Though - whether this would be a problem in practice, I couldn't say. Most software probably doesn't demand much. GPU-heavy stuff wouldn't be much affected. Emulated Photoshop would probably be terrible, but major vendors would get advance warning, I'm sure, so you'd have ARM-friendly Photoshop as a launch title.
And, better still, AArch64 is little-endian, and I think it handles misaligned data transparently (and if not, Apple could presumably fix that?) - so for 90+% of software, "porting" to this hypothetical future ARM OS X would require little more than a rebuild.
This wouldn't help Boot Camp much, though... I use Boot Camp quite a lot. Windows compatibility played a large part in my decision to buy a Macbook Pro in the first place! But I wonder how common that is?
You can bet Apple would work with select third parties like Adobe to port their software to be ready at launch if they were moving to ARM. They just did with touchbar.
Apple's done two architecture changes already. Both 68k backwards compatibility on PowerPC and Rosetta PPC backwards compatibility on Intel worked well.
