I agree with you, but Elon has been known to change his mind. Elon and others thought universal FSD would be a lot easier than it has turned out to be.This approach would go against everything Elon seems to believe in. He has been very clear that he is against geofencing or training FSD to be specialized in an area. Rather, Elon has said that he wants FSD to be generalized to work everywhere. So I doubt that Tesla will start curating data to specialize FSD to be a robotaxi in a specific area. Rather, Tesla will continue to gather data everywhere until FSD no longer requires supervision anywhere. It would actually be easier for Tesla to do what you suggest. But that is how Elon wants to do things and I don't see things changing unless somehow Elon did leave Tesla.
It's pretty clear that Elon wants to launch robotaxi soon. So I expect the desire to launch soon will lead to the question, "How do we actually get started?"
I'm just doing the thought experiment where Tesla decides to launch. How would that look? First, pick a city where FSD already works well. Second, make sure FSD works well enough to actually launch. If overfitting gets you there without appreciable harm to the ultimate goal of universal FSD, then Tesla is likely to take that route.