> but in reality the system is thoroughly backdoored on every level from the CPU on up, and everyone knows it.

Indeed. Why lock your car door as anyone can unlock and steal it by learning lock-picking?

There's a subtlety that's missing here: if your threat model doesn't include the actors who can access those backdoors, then computer security isn't so bad these days.

That subtlety is important because it explains how the backdoors have snuck in — most people feel safe because they are not targeted, so there's no hue and cry.