Everyone assumes a battery replacement, which may only be true of the Nissan Leaf. There is no battery replacement assumed for EVs like the Bolt or Tesla(s).
Here's a chart of degradation for the Model S:
In 17 years and 200,000 miles, we might expect the battery to be at 85%.
Interestingly, the degradation rate slows over time... although I'm sure at some point it accelerates again. There are no 15 year old EVs to reference as data points though. The oldest ones are 9 year old Nissan Leafs, and we already know they had degradation problems due to lack of active cooling.