Grid battery deployment is faster than EV market penetration in the USA. This is not something that can siphon off a fraction of EV batteries. It is a huge market in its own right.
What would be considered ruined Leaf EV batteries that have significant range loss (20% to 50% degradation, for a car that went around 60 miles with a full battery) are being used to cover peak load right now in California and have been used to do so for years.
But that isn't a mainstream case. It's notable for being weird. The 1000x larger Moss Landing battery facility just uses new, off-the shelf BESS units from LG.
https://www.youtube.com/watch?v=JqlOlqK_ot8
2nd life use cases for cheap, mass produced batteries like this are more common than you think.