The battery did not degrade to 50% capacity in 15 years, unless it was barely used and babied for the entirety of that time.

Capacity degradation happens over the life of the cell and you can see substantial loss in 3-5 years of normal use.

So when you're starting with a chemistry that's already poor on range, and then reducing that range by 20-30% within the first 5 years of it's life, you can see where customers would be unhappy with the performance.