EV batteries, just like the battery in your phone or laptop degrade over time from use and charging. The internal chemistry changes and usually yields one of two results depending on a number of factors:

1. The battery lifespan on a single charge becomes degraded. Sure maybe your EV had a 500km range when it was new, but over time, that decreases due to battery degradation. Thankfully, due to how many cells an EV battery has, the wear is more evenly distributed than in a phone or laptop, but EV's still lose 1-5% of total range capacity per year on average. So after 10 years, your 500km range may be decreased by between 50-250 km. (https://www.recurrentauto.com/research/how-long-do-ev-batteries-last) shows that around 8.5% of 2013 tesla model 3's owned by members of the site's community have needed a battery pack replacement due to range issues

2. Catastrophic failure chances increase as batteries age. This is due to several factors, but as age increases, battery temperatures trend higher for the same load, which can increase risk of failure. When an EV battery pack fails, it's very not good. Lithium ion battery fires are self oxidizing, they feed their own fire. You can't just spray it down 2ith water, it'll reignite. You have to let the fire burn itself out, or (as has been used for some fires) dig a trench, fill the trench with water, and then drag the car into the trench.

All this is to say considering the above factors, at a certain point, every current EV will need a battery pack replacement. With our current technology, the battery pack can cost between $10k-25k. When a 2013 Model S costs 26k used, I don't know that many people would want to spend $20k to replace the battery pack on it.