In the US at least, were it structured as an infrastructure project on a city by city basis, it would actually be extremely possible to strip out lead pipes and replace them with something modern.

The problem is that pipe ownership in a domestic home is the property of the homeowner, and there's no political will to do that kind of massive wealth transfer from the public coffers to the improved infrastructure of millions of individual properties. But while it wouldn't be inexpensive, it's on par with other expenditures the federal government makes for federal projects.