I'd imagine the Mac Pro will be the last of the machines to be transitioned to ARM, if only because it's the one that's been comprehensively redesigned the most recently.
In terms of new models per year, the iMac is where the churn is, the desktop environment is more forgiving than portables, and an iMac is what I'd expect to see first.
In terms of new models per year, the iMac is where the churn is, the desktop environment is more forgiving than portables, and an iMac is what I'd expect to see first.