Doesn't really make sense given their large existing investment in ARM and there being (afaik) no current work on POWER for mobile applications.
That's true and it'll likely prove to be a good business decision although not necessarily one that benefits the end user in the long run. I'm also not suggesting they use POWER for mobile applications neither did I suggest they use Intel x86 for that.
So POWER only for desktop systems, keep macbooks on something else?
Why would they switch back after spending so much developing their own ARM CPUs?
The way I'm seeing things, RiscV will be in the same business position ARM is in now, in about 5 years. Apple might switch to that, and if so I'd also expect no further migration after that (as it's an open architecture).
Apple might, but they probably won’t. They don’t have a strong history of choosing the open options over their roll-your-own solutions.