Lead poisoning probably did not cause the downfall of the Roman Empire
So often any of these single causal theories forget to look at timespan. Which was either decades or centuries... If system survived previously with mostly same things in place there is really no reason why such thing as lead pipes would be the cause of downfall part. By any reasonable conclusion the culprit in such case should be some disruptive innovation.In the end gradual natural degradation of system is much more reasonable.