Agreed that there's no prior precedent, but in the past, the move to x86 from PPC was universally beneficial across all product lines. There's no ARM that competes with mid or high-end Intel chips right now, and I don't buy for a second the complaints or suggestions that apple will abandon the pro market. Maybe it's a Rosetta Stone issue, or maybe it's a separate codebase entirely (a-la iOS and OSX) but there's no benefit right now to going all ARM, even assuming Apple's building their own monster chips.