It's extremely rare (if not completely unheard of) for a modern day car's brakes to totally fail. That would mean that both circuits had to fail at the same time (highly unlikely).
In such a "worst case" scenario... a manual trans car with totally inoperative brakes... my guess is that in 99% of those situations (rare as they are, probably actually non-existent), the average driver in a panic situation is better off mashing the clutch and yanking on the parking brake lever than trying to practice his F1 driving skills and downshifting to slow the car. Even the best downshift expert will have rear-ended that semi long before the magic of downshifting had slowed his car to any appreciable extent.
Bottom line: Downshifting to slow/stop a car in everyday driving is a completely silly and useless exercise. Sure, if you want to play Jackie Stewart, have fun... but the average person driving the average car in average conditions has absolutely no need to do so.