The impression I have: The first couple generations of Leafs don't have active battery temperature management and so far most Leafs that have entered the secondary market have done so in part because of large battery capacity reductions (necessitating in some of those cases battery replacements, which so far used dealers haven't been willing to finance at scale, so instead pass the "savings" to the next buyer).
As more Leafs show up with better managed battery lifetimes and/or as more used dealers get comfortable with battery replacements (and that gets subsequently cheaper as an industry), it may be likely that the Leaf loses its reputation as a "loss" on the secondary markets and maybe regains some of its resale value. (Especially now that current generations have more active battery management.)
But the entire secondary market for EVs is overall confused, because there are fewer EVs in the secondary market than should be. (The average first lifetime of an EV is way ahead of ICE "norms" right now, with average EV first owners keeping cars 7-10 years.) It may take used EV sales a while to better readjust prices to the actual marginal utility of a used EV (in light of overall reduced maintenance costs of a secondary EV lifetime versus ICE averages that used dealers have had decades of information about).
I definitely understand there is a snowball effect in place, but battery research (and as one example, GM's field testing) makes it pretty clear that degeneration occurs much faster when the Lithium-based batteries are used out of operating temperatures, and that appears to be reflected in Leaf used sales (areas prone to more extreme temperature ranges have worse used Leafs).
It's not entirely a fair apples-to-apples comparison, but GM's Volt has generally worse range as comparable Leafs at each early generation, and Volts are showing much less battery degradation when compared. (Certainly you may argue that the range extender of the Volt will bring down cycle counts, but even just comparing electric miles to electric miles the Volt is still outperforming the Leaf on battery degradation.)
As for the 20%/80& thing, that is something that for instance GM's tech actively manages and just bases its outputs as if the ~80% was "full" to avoid consumer confusion. The issue there is not that charge percentage matters, according to battery industry standards, but that individual cells have a "directionality" that should be respected and a discharging cell never used to charge until it has 100% discharged and vice versa. With regenerative breaking it is useful to have cells always available "in the charging direction". It is interesting that GM and Tesla took different approaches in marketing that to car users.
Looking at the volt, it seems to battery thermal management plus it only uses 65% of the battery capacity (similar to the 20%..80% tesla recommendation). I guess they can manage the battery capacity conservatively when there's a gas motor available.
Because after eight years, driving from Redmond->Seattle->Redmond with the heat on becomes a dicey proposition. Best keep a list of charging stations handy, that's all I'm sayin'. Used to be that trip was no problem if I kept it under 70mph on the freeway. Now I better keep it closer to 60mph, and leave the heat off if I don't really need it. The next gen used a heat pump and electric seats to keep you warm rather than an inefficient strip heater. (Digressive pet peeve: that heater draws 3K watts while it's warming up. Why the fsck does it take five minutes to produce heat? Where is that 3K watts going other than to warm my legs fifteen seconds after I hit the switch?)
And compared to what you can buy today, even if it's a used-but-later-model Leaf, they're weren't great when they were new. We bought ours because we have an ICE in reserve. Go buy a Chevy Bolt for the same money we spent and most folks can just forgo the ICE with the range the Bolt has. We keep the Leaf around because it suits most of our needs, and I usually get to work on a scooter or bicycle, so the wife can commute in it and the ICE sits. But as the battery degrades, he becomes more of a "running around town" car.
Guessing because when they were new, the range was marginal for a lot of uses (80ish miles), and uses drop off rapidly with even minor battery losses (when 40 miles each way drops to 25), your circle gets a lot smaller.
As more Leafs show up with better managed battery lifetimes and/or as more used dealers get comfortable with battery replacements (and that gets subsequently cheaper as an industry), it may be likely that the Leaf loses its reputation as a "loss" on the secondary markets and maybe regains some of its resale value. (Especially now that current generations have more active battery management.)
But the entire secondary market for EVs is overall confused, because there are fewer EVs in the secondary market than should be. (The average first lifetime of an EV is way ahead of ICE "norms" right now, with average EV first owners keeping cars 7-10 years.) It may take used EV sales a while to better readjust prices to the actual marginal utility of a used EV (in light of overall reduced maintenance costs of a secondary EV lifetime versus ICE averages that used dealers have had decades of information about).