All medical imaging software I know (except Slicer) uses LPS coordinate system in files. So, we don’t really have a choice and follow this convention to avoid errors and frustration of users.
To make the transition smooth, since 2017 Slicer writes it in the header of all saved model and markup files what coordinate system is used (LPS or RAS). When we change Slicer to start writing LPS files as default, all the new and old files can still be read correctly.
The final step is to change how files without coordinate system definition are interpreted by default. Old scenes will still be loaded correctly (because from the scene file version we know that the default was RAS back then). For standalone model files that were created by Slicer before 2017 the user will have to choose the correct coordinate system in Add data dialog manually. We can delay this final step for a couple of more years to give users more time to update their model files.