In both previous architecture shifts the new CPU family was a major boost over what the previous machines had. For instance, at the time the PowerPC 601 was first shipped (although it was not intended to be a production CPU and had been built as a proof of concept) it was the fastest microprocessor BYTE magazine had ever tested. A major jump for that era.
The PowerPC models Apple used also had the advantage of a dedicated bit of transistor real estate to do some of the conversions that would otherwise have added a LOT of overhead to emulating the Moto 68K family.
When it was decided to go to Intel the problem was that Apple alone wasn't a big enough customer for IBM to commit the resources in producing competitive desktop CPUs at a pace to match Intel. The profit margins were tiny compared to IBM mainframes but the level of capital required was greater. Apple had no interest in encouraging an open market for non-Apple PowerPC desktops, so something had to change. By the time the first Intel Macs shipped the last round of CPUs IBM produced for Apple were getting a bit dated by the standards of the PC industry. Once again, the new architecture had plenty of spare horsepower to handle the emulation problem.
Unless ARM's upcoming 64-bit product line offers unprecedented level of performance gain, at least an order of magnitude over current ARM designs, there is just no way to have a third relatively easy transition. Something has to give. Either Apple dumps its professional users and allows their desktops to age into uselessness, or they once again offer some hardware means to ease the transition.
I'd expect the former to occur as those users have become a very small portion of Apple's revenue picture. The cachet of holding that market is no longer as useful as it once was now that they've become so strong in the consumer end.
Another possibility is that Apple continues on x86 but dumps Intel. AMD is currently circling the drain but has some very valuable IP. Apple could buy the lot using the change under the cushions of the couch in Tim Cook's office. One question would be whether to retain the ATI portion of the company or spin it off into an independent company again.