When Tesla introduced the 70D and changed the order page to deemphasize the RWD version, I was of the opinion that it would remain available for many years due to it being cheaper.
But after reflecting on what Elon said last October about the Dual Motor version being just a better car in every way than the RWD (increased range, better handling, better acceleration), and noticing the trend of Tesla adding more features to the S without increasing the price much (former options like Yacht Floor, Parcel Shelf, Alcantara headliner, many items in the first version of the Tech Package, etc., now standard) I have come to the conclusion that in the near future the RWD version of the Model S will be discontinued, possibly even this year when the Model X launches.
Making the S available only with dual motors would simplify production, and that is something that Tesla has clearly been doing with the S over the past year, reducing the number of options and model variations. And it doesn't seem to have reduced demand at all. So why not?