I feel like an EV has an 8-10 year lifespan because after that, you better have the cash on hand to replace the battery.

Entirely true. But we can hope battery tech gets cheaper. But I'm not convinced that's going to happen. Especially with how fast battery tech is moving, it's not entirely unreasonable to worry about whether the battery will even be available anywhere when it does die.

Prices for useful batteries have already come down massively, to the point where lithium batteries are now growing into former lead-battery niches, because while lithium batteries are still more expensive, they are smaller and a lot more reliable. A lot of the research on battery tech is focused on using cheaper materials and on using less material per kWh. Cheaper LFP batteries are already complementing NMC batteries, and cheaper-yet sodium-based batteries are essentially in the stage of on-road validation. Even semi-solid state batteries are not too far out.

Also, by no means are batteries all dead after 8 years. There's a bunch of Nissan Leafs and Tesla S's to prove you wrong on that. It really depends on whether the batteries were fast-charged a lot and how much the car was driven. Also, it's worth noting that the car industry basically defines a battery as totaled if it holds somewhere between 75-80% of its original charge. By which point it is in fact not useless.