In a long time horizon, it's probably in Apple's interest to migrate away from ARM just so that they have one less company to negotiate with or receive restrictions from
Apple helped to design Arm v8 they probably have huge influence on the direction of travel of the ISA. Plus they have huge investment in Arm based software and probably get access to all of Arm’s IP (eg big.LITTLE).
They will migrate when there is a major performance advantage (as in the past).