They used x and z beforehand because the idea of using a “phone” processor in a device like an ipad may have turned people off - phone processors don’t have the reputation of being very powerful. And in 2020 they needed to say “this is better than 2019, because…reasons.”
Power users understand the difference between 12 cores and 6 cores. They don’t need, or care, about the chip having the same name. There will be M1s and M2s but no M1x’s or M1z’s. Mark my words.
Upon reflection Apple appear to be attempting to reverse to the perception that a 'phone CPU' could drive a Mac. They're first showing that a Mac CPU could run an iPad Pro which is also fair enough.
The A series CPUs appear to be remaining for (for now) for phones and consumer tablets which is fine.
But while the understanding that A15 will pilot the technology that will be emerging in the M2 - just with more compute and GPU cores and perhaps a higher clock speed - I would still still contend that unless the M2 is going to have a fixed clock speed and number of compute cores across all products (like the M1) Apple will need to figure out some way of differentiating M2 by core.
And then how would they justify an expected $3-5k iMac Pro up against a $1299 iMac for a regular punter who is used to knowing that i3 < i5 < i7?
Especially if Apple continue to omit MHz which is a good strategy (because punters immediately compare with other x86 CPUs without any context)
And given that A series CPUs debut every year can we assume that there will be an M series CPU on a fixed annual released schedule too? It would make sense if Apple don't want to rest on their laurels and need to keep the pressure up on AMD and Intel.