Pity ... RISC-V is so much better perspective ... and free.
RISC-V is not yet ready for the high end needs that Apple has. RISC-V doesn't yet have the single core performance nor the ability to scale to many cores like ARM does -- or at least I haven't seen it.
I think that when RISC-V can do this and is competitive with ARM, Apple could look at it. Apple isn't married to ARM forever, remember it used to be Motorola, then IBM PowerPC, then Intel x86 and now ARM.
Given that Apple just did the transition to ARM, I do not expect they will look around for the ARM successor for a while. But a decade from now it wouldn't be surprising for them to consider RISC-V, at that point its capabilities should rival ARM if not exceed it while also being much more easily customized.
Aegis CPU: Ascalon-based Chiplet System Architecture
From that same page.
What about the ISA would make Apple's core designs impractical or less efficient?
What it does is secure their ability to still use ARM for a while.