That was actually my thought for a while. Sooner or later there will be a massive amount of 'old' batteries from Tesla vehicles that upgrade to newer batteries. On a car it is critical to have the best weight to performance on a battery. Even a 10% capacity loss is considered bad. For grid storage, size and weight matters little. Grid storage batteries is a great use for recycled Model S batteries. A cell that lost 10% capacity is still perfectly fine in a grid storage system and the usage as grid storage is far less stressful on a battery than in a car. So they will probably be fine for many years to come.