That is unrealistic expectation for 15 years, especially for a 2017 EV.
It depends on how much it is driven of course. The mileage is probably a bit below average on that one. I have a short commute to work. We did some road trips in it though.
You gotta take into account that old EVs with small batteries will probably often be used as a second car, and so used less often. That’s the case for us since we got a newer bigger car as our family grew. We don’t take the Kia on road trips anymore and my wife takes the bus to work.
Don’t forget that there are other parts of a car that degrades as well with time. Eventually most well used cars will have an engine or powertrain issue if they even make it to 15 years to use without being totaled in an accident or simply sold second hand or misused.
For 2 reasons, primarily. First, Toyota chose nickel-cadmium batteries because of their higher charge-cycle life, at the cost of a much lower energy density (about half that of li-ion). Secondly, they designed their system so that the discharge rate and the discharge level of their battery pack stayed low, thereby maximizing the lifetime of their battery pack, at the cost, once more, of extra weight.
So those two aspects combined mean that you can expect a much longer real-life usage of your battery pack before reduced performance becomes an issue.
I fully agree that a battery pack replacement after 15 years could be considered reasonable (as long as the build quality of the rest of the car warrants it, which is not a given nowadays). But if the mean-time between replacement is around 5 years, then it becomes unreasonable.
I've seen >>10 year old laptops where the battery is still good enough to go from charger to charger. Just go to ebay and check out 2009 MacBooks. That's ~15 years now.
I don't think this is unrealistic if you can live with the heavier degradation.