Yeah... I don't know a lot about the calculations and whatnot, but even I know that 72 CU's at the proposed 2 GHz clock speed would be ludicrously powerful.
Still, maybe there's some room to play with the numbers? Are there any other CU numbers for a butterfly configuration that would work and allow proper BC? While I have no proof of anything, I would like to think Cerny might have been testing design ideas for the PS5 when he made the Pro. If the butterfly design worked there then why not on the PS5? Were there any real downsides?
My random guess for PS5 target is-
54 x 128 x 1900 = 13.1 TF
(54 is a nice number for back compat because x3 of PS4s 18 cus)