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

This is wrong assumption. FP64 usually uses the same circuitry as two FP32, adding not that much ((de)normalization, mostly).

From the top of my head, overhead is around 10% or so.

  > Why would gamers want to pay for any features they don't use?

https://www.youtube.com/watch?v=lEBQveBCtKY

Apparently FP80, which is even wider than FP64, is beneficial for pathfinding algorithms in games.

Pathfinding for hundredths of units is a task worth putting on GPU.