And it is also smaller. The ratio for the M1 versis A14 is about 36% ( approx 120/88 = 1.36 ) . The A15 is probably bit bigger ~98mm2 . If the MnX is around 220mm2 that ratio is 2.24 . That is substantively different cohort.
The volume doesn't as much as the wafer consumption. Volume contributes to that but so does size ( and defect rate ).
The other big problem was supply chain was not as out of kilter last year. That was a possibility at the outset ( something might go wrong with N5P that they may have accounted for and picked the safer option. )
Misdirection. You are the one who dismissed the any alternatives. " Why would it not". That is actually dismissal.
I agree with deconstruct60 that there might be other factors determining the architecture and node of MnX. The MnX chip is likely going to feed MBP, larger iMacs and perhaps a mini Pro. Taken together, I would not be surprised if these segments constitute a very large share of Macs and as deconstruct60 also point out, the chips are larger. So production capacity needed might be equal to the M1/A14 capacity.
The fab line could however be retooled. I was pleasantly surprised that the iPad mini got the A15 at the same time as the iPhone got it. So they went away from "iPhone first" policy that has hampered the iPad line for a decade. Marketing wise I think A15 based cores for the new MBP will make good sense and I really hope Apple upgrade their entire chip lineup every year to simplify it for the end user.
MBP and higher end iMac are less concerned about power draw* so I guess a 5 nm/A14 process and sufficient number of CPU/GPU cores will work as a charm.
*If you are using a MBP at 100% CPU/GPU utilisation, any battery empties in less than 2h, at lest if the machine is >2 years old. This will be better with ASi but you still need mains for high performance work.
We can speculate for ages until it is released.