If a car hit an actual brick wall, it would likely be closer to your 50 mph final reference frame scenario, with the wrecked car and wall remnants both moving n

If a car hit an actual brick wall, it would likely be closer to your 50 mph final reference frame scenario, with the wrecked car and wall remnants both moving near that speed. It would take some good anchoring for the ground to exert enough force on the wall to keep it immovable.