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.
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.