Also worth mentioning: The authorities already knew the subjects. The supposedly even had recent warnings regarding Charlie Hebdo. They have had police officers there for a long time, too. And for most cases of terrorism in the last decades it was the same.

Generally, the police usually knows about the suspects before. They just can't do much before they commit a crime. The unwarranted surveillance of everyone can't possibly prevent attacks by people who already were filtered anyway. Much better to invest the budget in classical targeted intelligence and police work and into improving the response _if_ something has happened.