That all depends on their current licensing terms, doesn't it?
Besides, ARM-to-RISC-V doesn't require a full redesign. Plenty of components are going to stay more-or-less the same, the big change is the instruction decoder. Chip developers have done far more drastic redesigns while staying with the same ISA - just look at the history of x86.
I think the bigger question is: does Apple want to go through another binary compatibility break?