STLs for subtractive CNC may not be good engineering practice, but it is quite workable as MeshCAM has shown:

https://www.cnccookbook.com/meshcam-great-ease-of-use-in-a-3...

I am pleased MeshCAM exists, I am pretty sure I will end up using it!

But I think we agree that mesh formats don't make good engineering interchange outside additive manufacturing.

Personally I think they often make a bad format even in additive manufacturing. They are the right format for medical/dental, and yes, slicers make internal STL-ish mesh representations of STEP etc. before slicing, but they are much harder to modify precisely than STEP.