Determined professional terrorists (like the ones who perpetrated the recent attacks apparently were) can always find a way around surveillance. For example, if two people communicated by having face to face conversations in random public places, the NSA would never detect them even if they had access to 100% of all electronic communications. During the Cold War, spies from both sides managed to operate for long periods of time in enemy territory using these kinds of low-tech communications channels.

And when that fails, as it surely will, do they get ever more powers?

Tell me again how those powers helped (or would have helped) with The Troubles?

The proponents of total surveillance never had a shortage of pretexts. And spreading the panic always helps.

Really? But why?

The cow is gone so and it is too late to lock the barndoor.