This will be the case in the next couple of years. Wall boxes are inherently smart (that is, controllable). So as a sink, the concept works somewhat already. But most cars won't be empty all the time. Also as source the 50kWh will only be usable as an emergency backup for the owner of the vehicle and only if they have the infrastructure to consume directed current. No one will let their own battery drain for the greater good. For public use cases people won't offer to unload more than maybe half of the capacity, realistically maybe 25% on average.