But take an even slightly more complex example: you're on a two lane roadway and the car in the other lane changes into your lane, leaving inadequate stopping distance for you. You brake as hard as you safely can (maybe you have a too-close follower, too), but still there will be a few seconds when you could not, in fact, avert a collision if for some reason the car in front braked.

I have no idea what the legal situation would be: is it their fault if the crash happens within 3 seconds but yours if it happens after you've had time but failed to restablish your needed stopping distance?

Honestly even in the simple one lane case, I doubt you can slam your brakes on the interstate for no reason then expect to avoid any liability for the crash, blaming your follower for following too close.

Driving has a bunch of rules, then an awful lot of common sense and social interaction on top of them to make things actually work.