Quite the opposite. Other parts of the car will wear out at 25% of that (or less), so you'll have second use batteries available with 75% of their life still in them. People won't want to build new cars with old batteries for safety concerns, but if these go into containers in a solar farm in a field somewhere, the risks are far less - you won't stress them as much in regular use, and any emergency will be more contained.

If they are able to be used in cars for a million miles, they will still be able to be used after that at a lower performance. It's likely that a used "million mile" battery will be even more valuable, since it presumably loses performance more slowly than other batteries. So at the point where it's performance drops below "useful in EVs," it will still have a ton of life left before the performance drops below "useful as part of the grid."