Obviously accidents happen. But I’d bet 100x over on municipalities (at least in areas I’d want to live in) over ordinary people any day. In places like Cambridge MA, NYC and SF the water is checked multiple times a day for a wide variety of contaminants using industrial processes unavailable to consumers. Similarly, they have the techniques, resources, knowledge, and scale to do filtration not feasible for ordinary people. Just looking at the wide variety of masks out there for Covid and their usage, assuming ordinary people would be willing to pay for top RO equipment and maintain it seems like a leap too far (most would just get a Brita and call it a day). It’s also unreasonable — we pay taxes in order to solve societal problems at societal scale using specialized knowledge and standards that few can replicate.
If there are cases where municipal water supplies are contaminated and it’s unknown for a while, then we need to address that by either changing federal laws, changing funding sources, or otherwise fix such issues for all.