The Model S is aging well (they really hit it out of the park with the styling back in 2012). But rather than see Tesla introduce a new model, I think it's time for the next generation of the S.
Car makers typically alternate between "all new" and "facelift". In Tesla's case, the facelift was the minor styling change they did a few years ago, which most noticeably got rid of the fake grill opening. So they've done their facelift - time to change the sheet metal.