The current rumor is Apple is going to ARM in 2020 for their computers. There is uncertainty if that will include MacBook Pros or Mac Pros initially or if it will just be their Air and maybe the MacBooks to start with. That's not to say they won't take their higher-end computers to AMD but I would bet if they are moving to ARM at all they are going to push for everything to be on ARM eventually and it's probably not worth the effort to switch from Intel to AMD in the interim.
As long as they're coming out with new hardware configurations anyway, why should switching to AMD require substantial effort?
There are modified Darwin kernels that allow Hackintosh to work on AMD processors. These kernels have some stability issues, but if hobbyist outsiders can get most of the way, I don't forsee it being a big hurdle for actual Apple engineers.
Because strategically, the move to ARM makes more sense for them to focus on even if sticking with Intel is a bit more painful in the short term. They already have a large team working on ARM processors and an architecture license for the platform. With x86, they are basically just resellers. So adding AMD's flavor of x86 to their lineup would likely be seen as a distraction for them without providing a long-term benefit.
Primarily, compatibility with legacy apps, and compatibility with other OS's (eg Bootcamp/Parallels).
I also have major concerns about raw performance at the high end, and I suspect ARM would come with even more software lockdown, although there's no reason that has to be the case.
I was watching highlights of WWDC and they mentioned that they're adding support to XCode to migrate iPad apps to the desktop.
I subscribe to the theory that the Air will move to ARM at some point. Adding this feature to XCode sounds like the sort of thing you would do to prepare the way for an architecture shift. Especially if you were still on the fence about that shift. Let's just get a feel of how viable this space is before committing to anything.
Except the change to XCode is a direct conflict of interest with moving MacBooks to ARM platform. If they are moving to ARM soon, there is no point in adding a brand new feature to the IDE that helps convert ARM apps to x86 apps. The reason Apple is doing so is due to the new Ipad OS that resembles desktop interface.
