I think the explicit button would be a more bulletproof solution, because if people cannot do a search for STL on discourse (there are at least 10 threads about this by now, so it’s easy to find if someone bothers to search), they won’t find the STL option in the save dialog either. The default saving option must remain the master representation, otherwise there will be bigger surprises (loading back will result in a different display and lossy data in the master representation). Saving the scene with both the master and STL could be a solution, but I think scene saving does not happen with the intent of using individual files from that in other programs (like for those who want do do 3D printing), but to load it back to Slicer later. Also, having “stray” files along with the scene that do not belong to any storage node is a loss of coherence in my view.
So in summary, I think STL is definitely an export type operation, and adding a button for it in Segment Editor would probably be both sufficient and easy enough to find.