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.

If you have to modify an STL, you're doing something wrong --- go back to the original/source file.

Well this is true, but it is STL files that people send around everywhere and publish in the 3D printing world, especially.

People will reasonably choose not to publish their original designs but can only publish STL because of OpenSCAD, Blender and because of the free tier of commercial CAD.

And having the OpenSCAD source doesn't help that much; you can modify it but you are still stuck with STL. OK so FreeCAD can reinterpret some — not all — OpenSCAD operations in a bRep context. But it's not a great way to work; it doesn't really help much with interchange.

I absolutely have had to modify STL files for people because that is the only interchange format the designer made available to them, or even could make available because they used the wrong tool for the job (like Fusion's free tier). At this point there are many STL-folk-song designs out there, CC-NC, with bad geometry, non-centred exports, etc.

Thankfully there are ways, in core FreeCAD and in a third party workbench (MeshRemodel), to more easily extract and reconstruct some key geometry from a mesh, such that parametric modifications can be made. (Whereas with OpenSCAD you're basically stuck with cut-out-and-roughly-refill).