Across America they have a mix of countdown systems (waiting and crossing), some are just countdowns for crossing and some countdown for both crossing and walking.

I agree that TfL have chosen one implementation method and there is little chance of them changing it to a better system in the next decade. But theoretically it would be possible to have a red man with a red countdown and a green man with a green count down. It wouldn't take long for pedestrians to realise that the red countdown is a waiting time rather than crossing time. And the implementation cost should not be much different to installing single countdown crossings.