The obvious reason is probably they want the older problematic models to be removed from the market?
If true, i think getting rid of the older classic Model S model from the market will be a big mistake and hurt Tesla's image.
Many early adopters have taken good care of their Model S and i think keeping them on the road in Public's eyes can only promote market confidence in Tesla seeing that those early models are still holding up well. Personally, i feel my 2013 Model S rides as comfortable or even more comfortable than the new models during my loaner experience with the 2019 Model s 75 and 2016 90D. i could be wrong, but the newer models with the facelift feel like a lighter car; the older models have a more solid feel.
It would be a shame if Tesla does not come out with an affordable battery replacement program for the early adopters in order to force us to turn in those older cars for newer models.