x86 will have have lasted 8+ years at Apple, which I think is reasonable when you consider how much Apple products have changed since x86 was adopted.
I also think a RISC instruction set is starting to look pretty nice, since (as I understand it) x86 chips break CISC instructions down to RISC-like micro-ops anyway. Why not move that process off the die (where space, power and thermal requirements are strict) into the compiler.
The transition away from PowerPC wasn't that tough in reality. If anything, it told Apple that such a change is very possible. With Windows 8 coming to ARM, it's not that big of a deal for Apple to follow suit.
But this was necessary to achieve any kind of real performance with x86. It's a terribly-designed architecture where all of the instructions are scattered all over the map. There's no unifying design, and most operations that should be simple to implement require scads of silicon. Much of the power wasted on x86 chips is in that instruction decode hardware. It's terribly inefficient. (It's also a horrible instruction set to deal with.)
Moving away from the X86 instruction set 5 years after the move PowerPC is NOT going to happen, period. ARM is fine for the iPad, but for high performance work it doesn't cut it.
ARM tech & Intel have proven that they can scale, but only Intel has the performance advantage for years to come.
And come on! Fragment the Mac market in laptops with ARM and workstations with Intel chips is a really bad move. The only laptop that would have an ARM chip would be an iPad with a keyboard.