First off, the silliness. If you think an iPad is a fully capable replacement for a MacBook, you must not edit many Makefiles. Paying extra for a TAB key just enrages some folks.
Some history. A friend worked at Apple back in the day, and his group produced a IIGS followon that was ARM based. Ran all existing (6502-based) IIGS code. Snappier GUI than the then-current Macs, cheaper, oops! So it was "gassed".
If you-all think that "just re-compile" is so easy, and Apple so supremely competent at re-targetting their software to new platforms, perhaps you can explain why their special flavor of X was so badly broken by the transition to x86? This was software that had run either (and even "cross") endian for over a decade and they managed to introduce rookie endian-bugs. Not to mention that even when they went from 68000 to 68020 they managed to stumble over the "let's just stick some unrelated flags in the upper byte of these pointers" bug that had bedeviled the 360->370 transition, again, a decade before.
Not to say it won't happen. They may be able to hire someone less Laurel-and-Hardy to do software (for a change). And the move to their own ARM SOC would indeed be a master-stroke for "you will get all your software via iTunes/App-store, and will update when we punch the button, and will not whimper or your device will die", which is so clearly the path forward.