We're at the severe end of severe?
And yet it *still* seems like the bulk of the terrorist plots are carried out by people who can't make things explode, or who are penetrated by the security services before they can actually do anything.
I can't wait until we're at the mild end of severe.
The only obvious argument for longer pre-charge detention is in the hope that someone who kept quiet for a month will somehow become talkative in the next fortnight, or to keep someone locked up who you're confident is guilty until sufficient evidence turns up to charge them.
Once someone has been in custody even for a few days, any co-conspirators still at liberty will likely have moved, changed identities, and altered their plans, unless they're extraordinarily thick.
If the bulk of the threat is from some kind of bomb or another, since there's no shortage of potential targets (basically anywhere with people gathered together), changing plans in the face of a potential security breach must be fairly easy.
If it's a matter of getting enough evidence to charge someone with something serious enough to warrant their continued detention, if you're desperate to keep them in custody because you're sure something is going on, you can always charge them with some kind of conspiracy and worry about the case falling apart months or years later at trial.
At the moment, it doesn't seem that there's much fallout if some grand terrorist plot trial turns into a bit of a damp squib. Unless such events become too frequent, I don't expect that situation will change much, and if there was some real emergency, most people would likely be even less concerned about the people who eventually have charges dropped or are found not guilty. It's hard to see what future circumstances would warrant an instant law change just to keep some dangerous person inside.