The security services here deserve a lot of credit for ensuring these events are as rare as they are IMO, but the threat will always be there. All it takes is for some impressionable waster to watch a few ISIS propaganda videos and he'll drive a van through a crowd of people thinking he's the 21st century Saladin.
Even if you totally crush ISIS, Al Quaeda, Boko Haram etc etc the ideology will still be there in small pockets like a cancer within the worldwide Muslim community.