What are you insuring against, exactly? If you pull the switch and thereby deactivate the immunotherapy, the patient will presumably die from the underlying disease. (It’s not realistic to think that such a patient could be switched to a different curative therapy.)
You'd be insuring against a safety event that could harm or kill the patient. Yes, once the drug switch is activated, I'd imagine there would be a switch to a non-CAR-T treatment. Why is that not realistic IYO?