If intelligence chiefs no longer than there is a critical and imminent threat of attack, then reducing the threat level from "critical" seems rather sensible. Then you can go back to the previous situation, where (with around three or four hundred arrests per year) over 99% of potential attacks are stopped beforehand.
It's almost certainly impossible, sadly, to stop everyone who wishes to kill others indiscriminately, since, as we've seen recently, it's enough to drive a car or truck along a pavement and you don't necessarily need much planning for that. But as a matter of resource management, keeping on 24/7 high alert in preparation for something that may happen today but almost certainly won't, especially if you don't know what it is, who will do it or where it will happen, is a waste of time, money and effort. And it might happen anyway even if you did try to put officers on every street corner. What next for the escalation of security in that case?
I was going to start posting a slippery slope about how next you'd lock up anyone who could potentially be a terrorist, and then realised that people on AB have literally already called for exactly that. Thankfully, people on AB have no power whatsoever.