Unless the French intelligent services were picking on people at random they already had information on the people they should be watching as evidenced by the immediate sweeps in the aftermath.

In many of these attacks the individuals are well known to the police and security services as they often have criminal backgrounds and have been in prison or custody of some kind. So either they are some sort of criminal masterminds, which I doubt, or the security services are completely inept even when they have a wealth of information or something more sinister.

So in my mind more intrusive intelligence is not really the answer.