Not very long after 9/11 a thread was running, as you can well imagine, which discussed in detail the vulnerability of the passengers waiting to check-in and also queuing to go through security. Many of us have stood in those snaking queues with perhaps 300-400 people packed in close proximity, STN being one of the worst I suspect. Once our unprofiled terrorist reaches the middle of the crowd, pop, no more crowd. A similar situation appears to be the case here, only difference is it was in an arrival area.
There will never be a solution to this problem unless we stop people flying.
To prevent this do we move the security queue outside the terminal, down the road, outside the airport? If hundreds of people want to leave an airport at or close to opening time, being the busiest time, then there will be crowds of people. Same goes for the Tube stations and bus stations. Sadly, now that suicide bombers have the misguided notion that they will be martyrs in heaven the instant they press the button, these crowds will always be extremely vulnerable.