Believe that Tesla sees their future more in the Model 3, Y, Truck, Pickup, etc.
S & X will soldier on, with constant improvements and updates, but don't believe that doing a new version of them would really bring much more volume or profit. With the release of the upcoming Plaid versions with 3 motors and even higher levels of performance, that will probably bring in just as many new sales as a revamped body style. It will entail a wide body modification, as well as changes to the suspenstion etc.
Since existing models are already equipped with the items needed for upcoming FSD, that also can just be updated to provide for the newer technology.
They are still the fastest, quickest, longest range powerful long distance cruisers in the world.
Hard for me to see a reason to change them with whole new models.
Reality is that sales are not low. They are among the most successful and highest production EVs in the free world. Only seem small because of the run away success of the Model 3.