And for a lot of the older Mac developers, iOS was kind of like another migration. And iOS has gone from armv6 to armv7 to arm64, plus the x86 and x86-64 simulator targets. Not to mention that there is now an LLVM Bitcode requirement for Apple TV and watchOS.
Apple and its developer community has a lot of experience with architecture migration. Each transition built on the experience of the previous and got smoother each time. Apple has been very good insulating their frameworks and tools from the architecture, and the Apple developer community has gotten very good at following Apple's guidelines to minimize disruption since there have been so many of these transitions.