That's not correct. Cars will generally have something that's often called a 'limp home' mode. I saw a car that's PCM (powertrain control module) that failed some internal test, so it fell back to a basic mode where the engine wouldn't rev over about 2000RPM, and I'm sure all the emissions systems went into a basic fail-safe mode, where things like the fuel delivery goes into a hard-coded mode, instead of using feedback from the oxygen sensors to tune the fuel delivery. But a loose fuel cap doesn't disable the ABS or TCS. Even a single faulty wheel speed sensor doesn't have to disable the entire ABS, it can still independently assess each wheel, determine if a lock-up is imminent and modulate the brakes for that wheel.