At some point, I expect Tesla will add battery replacement/upgrade options - possibly only for S/X cars initially, to provide increased value for the S/X cars vs. 3/Y.
Once Tesla starts doing battery replacement/upgrades, they may be able to refurbish the returned battery packs, and get them operating well enough to provide them as a lower cost option for the replacement/upgrade than buying a new battery pack.
For example, when a 90 owner upgrades to a 100 battery pack, Tesla could refurbish the 90 pack to get back most of the range, and then sell the refurbished pack to a 40, 60 or 75 owner at a lower price than a brand new pack, with a reasonable warranty.