I kind of wonder if the tipping point of "exponential decay" might be where a battery starts being charged and discharged more often to reach daily range and maybe outside the 20-80% window you need for a healthy battery.

In other words, if a battery is new and has 200 miles range, but is driven 100 miles a day, it will stay between 20-80% charge each day when charging.

but at 80% capacity, 160 miles range, it must be either charged above 80% or discharged below 20% each day which is unhealthy for the battery. (either 80%->17% or 83%->20%)

as soon as it starts getting out of healthy range I can see how it can degrade faster.

But cars that stay in that range will have a much longer lifespan.