So batteries on EVs generally don't fail dramatically, they slowly lose range over time.
So someone buying a used EV will presumably buy something with enough range for them to get around, and then over time if they notice the range starting to decline to the point that it might be an issue, they'll have some time to save up & trade in their car for a better one. Their current car won't be worthless on the used market and someone who doesn't have as large of range requirements (e.g. a 2nd car for commuting) will still buy it.
There are other issues that can prevent an EV from charging at all, but they have usually cheaper fixes than a whole battery pack replacement like you're talking about.
So someone buying a used EV will presumably buy something with enough range for them to get around, and then over time if they notice the range starting to decline to the point that it might be an issue, they'll have some time to save up & trade in their car for a better one. Their current car won't be worthless on the used market and someone who doesn't have as large of range requirements (e.g. a 2nd car for commuting) will still buy it.
There are other issues that can prevent an EV from charging at all, but they have usually cheaper fixes than a whole battery pack replacement like you're talking about.