Monitoring the internet with programmes like Echelon is surprisingly easy. Millions of people can be survielled simultaneously without anything like the drag that is imagined. The task becomes resource intensive when you go from remote monitoring to tailing suspects
I think we also need to remember that some of these plots we're forever being told about that we've foiled are incredibly amateur. Some of them have been reported by court correspondents with restrictions lifted, and honestly, you'd back Inspector Clousseau to solve them. One guy enter a plea in mitigation that he was trying to get caught. Well when you look at what he did, it seems plausible. He used a twitter handle called 'mystery bomber', adopted Mohammad Enwazi as his avatar, posted youtubes of his experiments with explosives in his back garden, and then finally decided to organise an online poll for his target choice.
Something else that perhaps people don't appreciate, and Grasshopper has touched on it, is that we need to think a bit outside the mosque (so to speak). Even when I was doing a bit of early days 'prevent' stuff about 10 years ago the focus was shifting away from mosque monitoring. If you go into a mosque today and run your mouth, you'll likely be reported.
Even 10 years ago we were finding things like martial combat and boxing clubs, youth distraction work (7/7 bombers were recruited this way) badly monitored and erroneously supported community projects (often with education slants), outward bounds centres, or even public parks (Charlie Hebdo attackers) are just as likely to be the source for meetings, plotting, and making introductions. I think we also have to throw prison into that mix too. Much as though it pains me to admit there's a grain of truth in what the grey lady says, there is, even if she over-states it, and even if she's trying to use it as a vehicle to control what we see and consume (so no more porn and plenty of Abba instead). Having said that, today's backdoor is tomorrows exploit, as the recent round of ransomware demonstrated given that it started life with the NSA.
One day we're going to come up against a well resourced, well trained, and well executed terror cell. Only then will we know. What will happen eventually is that the public will begin to lose confidence in the authorities to defend them and you hardly need to be a genius to work out where that ends up