> Second, The likihood that anyone has the time and energy to go and build ECU software and flash it into their car is low.
I don't think this is true. There are plenty of enthusiasts who already do exactly this kind of thing. A friend of mine had an aftermarket ECU installed in his Mustang a few years ago. There's also the (idiotic) phenomenon of rolling coal[0], which also requires at minimum altering some engine tuning parameters in real time. Hackers abound in all walks of life, and this includes auto enthusiasts.