Based on articles I read in trade mags via my work email is that battery manufacturers suspect that with careful design and thermal management a lightly used automotive battery could have a 20-40 year service life.
I think people have a disbelief that there have been large improvements in battery lifespan in the last 20 years when there absolutely has been. Modern ion batteries aren't just 20 year old ones but cheaper. Degradation and charge discharge rates are substantially better.
