Well, as to the overall decline of the empire, it was happening with or without the plague, or the lead. Inevitable.
As far as the lead, it carried water and was used for eating/drinking utensils. That had to screw up the people using it.
I’ve got more lead in the water here than the Romans had, from old solder, and I’m in no danger. The risk of lead pipes and lead drinking utensils were known to the Romans, and their water pipes were 99 percent clay tile, with some lead and bronze used to supply the baths.