A few things switching to Arm could do for Apple, allow Apple to better compete on price, and kill the clones( hackintoshes ).
The first, Apple really has never wanted to compete in the low-end value market, they'd rather be a status symbol and command higher margins. Not that Apple Arm Macs would have to compete solely on price, they are still the only supported way to run the macOS.
I just can't see the top end of the "Pro" line moving to Arm anytime soon, but these Arm switch rumors don't seem to go away, so where there is smoke, there tends to be fire. The Mini and the Air, then the low-end iMac and 13" MBP, may well be slated for a switch to Arm.
I'd say the 16" MBP, iMac Pro, and Mac Pro, will remain x86_64, but for how long, Apple can throw tens of billions at Arm fab until they have a CPU that can run code near the same speed as x86 when natively compiled for Arm.
It's just, does Apple want to be in that game, they'd be putting themselves into direct competition with Intel and AMD as far a desktop and mobile CPUs, yet only be selling them to themselves.
I think, as long as they can take some form of the iPhone/iPad Arm CPU, and for not too much money on R&D, put that in a consumer Mac, people will be happy with that, but "Pros" are not the same market, and I don't think Apple really wants to put money into fab of Arm for desktop to compete with x86, clock for clock, thread for thread, and watt for watt.