> Assume FP64 units are ~2-4x bigger.

I'm pretty sure that's not a remotely fair assumption to make. We've seen architectures that can eg. do two FP32 operations or one FP64 operation with the same unit, with relatively low overhead compared to a pure FP32 architecture. That's pretty much how all integer math units work, and it's not hard to pull off for floating point. FP64 units don't have to be—and seldom have been—implemented as massive single-purpose blocks of otherwise-dark silicon.

When the real hardware design choice is between having a reasonable 2:1 or 4:1 FP32:FP64 ratio vs having no FP64 whatsoever and designing a completely different core layout for consumer vs pro, the small overhead of having some FP64 capability has clearly been deemed worthwhile by the GPU makers for many generations. It's only now that NVIDIA is so massive that we're seeing them do five different physical implementations of "Blackwell" architecture variants.