Apple's not switching Mac CPUs because of performance, or even cost, but primarily because of vertical integration. They're no longer slaves to the whims and failures of Intel, and they can build chips that fit the kind of Macs they want to offer exactly.
So they'd never switch back again, because then they wouldn't have the tight control anymore which they desired (and achieved in their iOS devices) in the first place. It wouldn't matter if one or more generation of chips deliver disappointing performance; Apple's Mac ecosystem is its own ecosystem and has never depended on cutting edge performance anyhow. It has always been user experience first, everything else second, third and so on.
So customers would continue to buy what Apple's offering them, just as they always have been. Not that there seems much risk of Apple screwing up right now, really. They've been owning the ARM SoC marketspace for years now in both in terms of absolute performance and power/performance ratio.
What's more interesting (although food for a different topic really) is how exactly they'll migrate their iMac Pro and Mac Pro product lines. Mac Pro tops out at 28 cores; such a chip requires the biggest R&D investment but also sells in the smallest numbers, making it difficult to earn back money spent on its development. Perhaps Apple will simply go back to multisocket chips like in the Mac Pro G5 and up until the Trashcan. So iMac gets the 12-core A14Y processor or whatever they'll call it; Mac Pro gets 2x 12-core processors. Something like that. *shrug*