While I totally agree with the sentiment of your post and hope that RC better supports adaptive decimation, one workaround to the “no way of saving a selection” issue I can suggest is the following:
- adjust the reconstruction region to include/exclude a region you wanna decimate
- export the reconstruction region
- select the faces instead of the region by going to Tools > Mesh Model > Box > select triangles inside the box
- simplify to X amount
- import the reconstruction region
- select the faces OUTSIDE of the region by going to Tools > Mesh Model > Box > select triangles outside the box
- simplify, etc