There will always be a question of backward compatibility due to the nature of the Mac ecosystem that you don't have in iOS. Then you have the issue of getting developers to migrate any existing arch specific optimisations. This can be mitigated somewhat and Apple has done this a few times before, but it's still a massive undertaking.
It may be easier for them to play the CPU vendors against each other if there is real competition in the AMD64 space. There are rumours of Apple going ARM on Macs, so it may be that that path is set, but if it isn't, there is sense in holding out the transition to ARM at this time.
Honestly, Apple has killed backward compatibility multiple times, first from PowerPC to x86-32 and x86-64, and now, killing x86-32 in Catalina.
Apple also has a good solution to this problem (the AppStore solves it), and a big interest into locking developers into this solution...
So Apple doesn't need to restrict themselves to any hardware vendor when it comes to CPUs, and due to Metal, they don't have to do that either when it comes to GPUs.
It may be easier for them to play the CPU vendors against each other if there is real competition in the AMD64 space. There are rumours of Apple going ARM on Macs, so it may be that that path is set, but if it isn't, there is sense in holding out the transition to ARM at this time.