Apple holds a rare Architecture license to the ARM ISA due to their role as a founding partner, and can make original implementations without incurring licensing dues, as they do with the A and M series CPUs. I understand why they would want to switch to homegrown designs instead of licensing ARM reference designs (which they do have to pay for), but why migrate instruction sets too when RISC-V doesn't save them any marginal cost over novel ARM designs that would be binary compatible with the existing firmware?