Interesting concept, and interesting point.
Assuming there is a law preventing Tesla from selling a used part in a new car, as I expect there may be, I wonder if they'd be allowed to offer the following, if they had an excess of 85s, (or whatever the battery being turned it at the time happens to be. )
A customer purchases a new 70, but is given an option to accept a used 85, --NOT SOFTWARE LIMITED TO 70-- at no extra charge.
Assuming Tesla would be allowed to do that, it winds up being a great use for the turned in batteries, as Tesla then manages to get excellent value back on them. The only "cost" is any incremental revenue not received from a customer who would have spent the extra money to get an 85, but gets the upgrade "for free" this way. One way Tesla could minimize that would be to not offer the option to everyone. So the only way to be certain you'd get an 85 would be to buy an 85. If you ordered a 70, a couple of weeks before delivery you may or may not get a call, asking you if you want a free upgrade to a used 85.
There may be legal issues with this, but if those could be dealt with, I think a program like this could actually make sense for Tesla. It might allow them to charge less for battery upgrades when the used packs are turned in, which could in turn help increase demand for the cars if people see a more reasonable way to be able to upgrade the packs to larger packs down the road.