My MG may be an ultra-simple car, but unless I lose my hearing or sense of smell, I won't forget it's running. Heck, because of the design of the door latches, it is impossible to lock the keys in car; the lock lever won't move unless the door is latched. I have to close the door and lock it with the key from the outside. Sometimes simple isn't so bad after all.
CO poisoning isn't the only thing you have to worry about; Even if your cat is working properly, it will still consume oxygen and put out carbon dioxide. You can still be asphyxiated.
I'm not a fan of these pushbutton cars. I've driven a Prius and it's easy to forget to push the button to turn it off before getting out, especially since the engine may not be running.