Undoubtedly true of a large system. But what about a small system serving say ten to thirty thousand people? The dilution problem wouldn't be nearly as bad as in a system serving a million people, the system is more likely to have a single point of vulnerability, and the operators are less likely to be able to deal with the problem even if they do discover it.
The "take" of infidels would be lower of course, but if coupled with "credible threats" against larger systems, panic could indeed be a problem.