Okay, you're assuming (correctly) that there's no way to concoct a system that will prevent all unwanted fires from getting started. When Zizek makes the kind of statement you take issue with, he does so starting from the premise that a vastly superior system (in terms of fire-related deaths and injuries) for preventing fires is indeed possible to develop. The reason he (correctly) assumes that such a system cannot be implemented alongside reasonably efficient squads of firefighters is because there needs to be a very strong incentive before people will awaken to the realization that systemic change is vital, even if the powers that be don't want the change to occur as it doesn't disproportionally benefit them like the current system does.

By the end of the video he's already nuanced a lot of his rather radical claims. He knows full well that change isn't as simple as either letting society rot sufficiently, or working patiently and diligently to affect gradual change in the current system until it somehow is transformed into the best possible one.