Their design is basically gated by the number of memory controllers: 1MC tops out at 16GB (M1), 2MCs for 32 (Pro), 4 MCs for 64 (Max), and I guess 8 MCs for 128 (Ultra which is apparently two Maxes stapled together).

Hopefully the next gen will provide more capable and flexible memory controllers, both so they can scale the top end for a full Pro-scale offering, and so there is more memory flexibility at the lower end e.g. the ability to get an M1 with 32+GB RAM, or a Pro with 64+.

They mentioned the Mac Pro replacement is still to come.