Watched an interesting video of a degradation test on a 7-year-old Tesla Model S. It is a P85, one of the first 6 Model S's in Norway. After 7 years the battery has only lost 5% and will still return 275 miles on a charge. The guy says that the degradation is a bit like the depreciation of a car. It is high in the early years and reduces as the battery ages. He reckons that after 20 years, the battery would still have 90% capacity.
I think they have been genuinely surprised by the longevity of the batteries, particularly those cars with proper battery cooling systems.
I can see a healthy market in battery pack reconditioning developing, just like engine and gearbox reconditioning specialists exist now for ICE vehicles. You can already go to a specialist and have all the faulty cells replaced to extend the life of failing packs.
Once the supply of new batteries is in full flow, you might even add to that the availability of aftermarket replacement battery packs.