[fsloth]

"financially viable lightweight enough"

That's the catch, I think. The market is not so large for 3D modeling tools.

The second hurdle is the ridiculous amount of inertia DWG has in the AEC industry. Any solution that would topple Autocad would need to likely provide DWG and/or Revit interoperability to even have a shot at the market.

There are no good options for Revit at the moment so, that's a deep vendor lock in. For DWG there is at least Teigha library from ODA which is based on reverse engineering the DWG format and offers reading and writing of it.

[rurban]

And there is the revived GPLv3 LibreDWG, which can read most DWG versions. And write support is pretty close. https://savannah.gnu.org/projects/libredwg/