Many people here seem to think Apple is capable of immediately taking the desktop CPU performance crown from Intel. There's a good chance that they wouldn't even manage to compete with AMD.
Many also seem to think that CPU performance has stagnated. That's not really true. Single-threaded performance has steadily improved year on year by 5-10%. Energy efficiency has also steadily improved. For the 'mainstream' platforms - i.e. iMacs and MacBooks, it's true that Intel hasn't delivered game-changing products (until 8th gen with quad core CPUs suitable for the ultrabook form factor). Look at AMD - even factoring in Ryzen, their year-on-year gains are no better than Intel's - and you can bet that both these companies are working very hard to uncover additional performance - especially single-threaded.
The real truth is that the low hanging fruit has disappeared, and getting these single-threaded gains for *high performance* CPUs is very difficult. You can't simply add more cores - you have to have an interconnect, memory bus, cache design etc that allows you to scale the core count. You can't simply add more frequency, because that increases the thermal envelope, power usage etc.
Getting CPU gains for the low energy ARM CPUs has been a different story - there was plenty of low-hanging fruit to tick off, and as a result we've seen multiple companies make huge strides (Apple, Qualcomm, Samsung among others). The signs are that this rate of improvement is also starting to slow down.
Now - instead of looking at mainstream consumer products, look instead at the high-end desktop and server segments. Total compute power per socket, along with bus bandwidth, has improved *dramatically* in the last 10 years. Where once we had 4 cores per CPU we now have 26! This is one of the areas where Intel (and AMD) have substantial design experience and patent warchests that make it difficult for other designers to compete.
CISC vs RISC isn't really an argument here either. X86 has some baggage, but modern X86 CPUs get rid of that pretty damn well, and it's very difficult to argue that modern ARM is really a RISC architecture and doesn't have its own oddities and baggage. Certainly on a laptop or desktop the 'X86 tax' isn't going to be that significant.
As for any actual transition to ARM, a non-x86 Mac would almost certainly kill my interest in the Mac. Apple have done a pretty good job of pushing me into a corner with their decisions of the past 5 years, and even without an ARM transition it's far from certain whether I will ever buy another Mac as my primary machine (after always owning a personal Mac for the last 15 years).
PPC to Intel had real benefits, and even then it had a significant amount of pain with older software and Rosetta. Intel's chips were hugely more performant than IBM/Motorola's, which reduced the pain a lot, and Apple was able to immediately take advantage of better interoperability. Boot camp. Virtualization allowing VMWare Fusion and Parallels. This was *huge*.
What is the benefit to moving to ARM? Better battery life? Unlikely to be much. Better performance? Unlikely. Virtualization of ARM? Well, iOS is the closed platform - developers already compile iOS apps to x86 to run them in the simulator - it's easier to make iOS work on the X86 Mac than make the rest of the world work with an ARM Mac.
If this really is Apple's plan, Cook is going to return the Mac to it's NuBus days. Yuck.