"..a risk of violence to the whole of the population"?
How many Lawrence like racial murers do we really get?
How many actual people actually were killed on 7/7
How many more would have died in the suicide attacks on 21/7 but for the lucky chance that the explosives had degraded?
How many next time?

I fear you may be seriously underestimating the threat.

That is what those who we pay to know these things are telling us.
I think we should listen.