What nonsense. If monitoring is meant to stop terrorism but terrorism continues it is obvious that monitoring has failed to stop terrorism.
The original statement whether monitoring had any
capacity to stop terrorism not the absolute
capacity to stop terrorism. You are quite correct that monitoring has not shown it has the absolute capacity to stop terrorism. However, if there is just one incident where monitoring has stopped an incident of terrorism the statement is proven in contradiction to your QED. Until such time as anyone can advance any definitive proof that monitoring has never stopped any incident of terrorism then your QED is just waffle. Quod Erat Demonstrandum (that which was to be demonstrated) has clearly not been demonstrated as we have no way of knowing.
If you would prefer to debate the statement "Only if it had the absolute capacity to stop terrorism. As it doesn't, it's monitoring for the sake of monitoring." instead, then we can all see that your QED is quite correct.
So the question comes back to whether society as a whole is safer with some sort of monitoring and if so, what should be the boundaries? If we could have a sensible debate about that rather than scoring cheap points backwards and forwards, we all (myself included) might have a better understanding and a view about the best way to go forward.