Rollbacks may make sense if the standards are too stiff about things that aren't as harmful as we might have thought in the past. A rollback may be tolerable...that is a separate issue to discuss from pollution generally. Having a view on pollution (or "plastics") has nothing to do with the highest level of water quality in the US.
The problems we see in the United States water supply are not in the places that might be affected by a rollback--the places that have the purest water. The problems we see in the United States are the places which cannot comply with the standards. We need major investment in places that have bad water. Focusing on the targeted standard is less effective than focusing on getting the worst places compliant with a higher percent of the standard.
Strangely, almost all US water, even the bad stuff, is better than water in almost every other country.