and I suppose all the staff should wear identical uniforms and airports should be made to look like towns etc etc.........
There has to be a point at which the measures proposed to counter a threat are impractical and I think your suggestions have reached it. Yes it might happen but then so might a train be blown up as it leaves a crowded commuter station, or a ship be attacked as it left port etc etc. We are all hoping that the intelligence forces are now good enough to forestall any of these types of attack, but your suggestio doesn't actually prevent an incident it just makes it more haphazard.