I notice this is specifically for Toyota Hybrids. Of course there are a lot of these on the road so after market providers have a big customer base. It may be, in time, a similar set up arises for EVs but, at the moment, a lot of people seem to be putting up with the loss of battery capacity. The value of a car with lost capacity drops like a stone. I'm seeing some Leafs at around the £3,000 mark now - not many and not at dealers but shows that an old Leaf is not worth a lot.
One thing that I have gleaned from lurking round the EV scene is that Tesla batteries seem to be holding up better but they, unlike the Leaf, have active battery cooling.
The advice seems to be that you need to build in battery degradation into any purchase so that probably explains why most Leafs are bought on PCP deals. If you are buying second hand the advice seems to be to ask yourself how long you want to keep the car and whether you could still cope with, say, 75% capacity after 6 years or so. This means a car that could do 80 miles would be down to 60 and, possibly, below 50 in bad weather.