It’ll happen if and when Apple starts to feel that macOS on 8GB of Ram might begin to become unusable.
they kept a 5400 RPM hard drive in the iMac until 2021, 5 years after APFS (which was literally built for SSDs) had become standard in macOS.
They will keep that standard until they are just forced to change it by everything else.
A better question is, will they do a half step? The M2 introduced a 24 GB of RAM option, could the M3 introduce a 12 GB option? I think it’s possible, the iPad was on 6GB for a couple years so it’s not like this would be the first time.
So I do think that is a possibility, systems starting with 12/256 later this year.
16/512 I think is still 2 to 3 years away, maybe the M5 or M6