That is why you are supposed to shift your weight backwards to reduce the chance of going over the bars. It is about how hard you have to squeeze to achieve a lock up and how much retardation that can be achieved before the wheel locks, not how controllable the bike is when it's locked up. A locked up wheel has less grip than a rolling wheel and gives less retardation. The fastest way to slow down is with the primary effort through the front wheel, end of story, the mass of rider and bike are thrown onto the front increasing the effective weight through the tyre onto the road and increasing the contact patch and therefore the grip. Watch some moto gp or superbikes, they brake into corners with their rear wheel off the ground and that must be the fastest way to slow down otherwise they wouldn't be doing it and they are operating under the same laws of physics as everyone else.