(ETA: It also caused lead to be released- non-lead pipes had actually leached lead from the water over many years, acting as a filter. The lead was chemically bound to both lead and non-lead pipes. Changing the chemistry caused the process to reverse and dump all that lead back into the water at much higher concentrations than would occur just using a 100% lead pipe system.)
And lead paint is similar. In-situ it's pretty harmless. It's the removal that's dangerous and is more likely to poison.
This is why people arguing for "do-something!" actions using "What harm could it do?" reasoning should always be fought until safety is confirmed experimentally the field.
"What harm could it do?" should always be a red flag for "don't know what you don't know" ignorance, and responded to as if it's a real, non-rhetorical question.
This is not completely true. I remember some studies looking at lead accumulation in dust. It's not just macroscopic paint chips on the outside of houses that children ingest, it's dust in the house as well.
Just replacing the pipes seems like the optimal solution, but half-measures are better than nothing.
All it takes is a change in ph of your water supply to put you at risk.