Even looking at the recent Westminster attack, is it realistic to think that monitoring of his WhatsApp messages would have prevented anything from happening? W

Even looking at the recent Westminster attack, is it realistic to think that monitoring of his WhatsApp messages would have prevented anything from happening? Was he already under surveillance? Could the attacker really have written anything so specific and unequivocal that it would have actually made police go to Westminster and stop his car? Police with limited resources can't just go investigating everyone who sends messages, so I doubt it would have helped anyway. I hope you're right, but I don't think so. The next time there is an attack, I don't think there will be an outcry that the all-pervasive surveillance has failed us, only an outcry against the terrorists (who, let's face it, are the real offenders).