Do any of Apple's laptops even have more than 4 cores?
I can absolutely see the MacBook going ARM in the mid-future. Despite it going against what Apple's been saying, there's no way Apple doesn't have a long term iOS/macOS convergence strategy.
I think you can take them at their word that they don't see iOS and macOS merging any time soon, preferring different form factors for different work (and getting you to spend more money on more devices), but there's no doubt a room with Macs on ARM or AMD processors or whatever frankenbeasts they can envision. Apple has gotten burned enough times on stagnating chip performance they're going to be ready to make a shift if necessary.
Honestly, though, I don't see them only transitioning one of their computers over. They'd do the entire consumer line at the very least or not at all.