I could be wrong on the facts here as I have not been following the story closely, but I believe that the unencrypted SMS found on a phone outside the Batcalan helped lead police to St. Denis where they killed the mastermind and several other terrorists and likely prevented a further attack. Between St. Denis and the raids in Belgium, it seems like the police likely (1) already knew a lot about many of the people involved and/or (2) were able to quickly piece together clues, both of which were probably aided by the ability to monitor digital communications. Look also at the number of targeted killings of terrorists over recent years (e.g. Jihadi John) which I would assume were aided by digital monitoring.
And the terrorists are aware of the above and are now training their followers to use apps like Telegram.
With 7 billion people in this world, preventing every single attack by mass surveillance alone is impossible. But I think its naive to believe that surveillance is not helpful to law enforcement or that the pro surveillance crowd is motivated by some sort of ulterior motive to blackmail people. If you are the head of the CIA or NYPD, you probably spend a lot of your time genuinely worrying about letting an attack happen on your watch and you would be strongly inclined to fight for every tool you can get.
The question is whether the benefits of dragnet surveillance and weakening encryption outweigh the negatives, and like others here, I'm inclined to think they do not.