Reducing head way to less than stopping distance

With the advent of communications-based train control, why is the moving block system being used when even shorter headway can be achieved? For example, if two trains are going the same speed and have the same braking characteristics the headway only needs to be the response time plus uncertainty.

If the leading train has a derailment, it will stop faster than the following train can stop, leading to a collision. What is really gained by operating without a safety margin?

There would have to be some system where if an emergency, or hard service brake application happened on the first train, the system would automatically do the same on the second train. The engineer or operator wouldn't have the reaction time to stop. This would only work if both movements had exactly the same braking. What if the second movement had more warn brake shoes? What is the second was heavier, due to more passengers?

... and the passengers aboard the derailed train would be thrown around quite violently. You'd have to do the same to the passengers aboard the following train in order to avoid colliding with the derailed train--in fact I don't know that you could even stop the second train as quickly as the derailment stopped the first one.