the "more democratically controlled" members of government are congress, who make the laws, and can make more laws. That's their job – not anyone else's.
Expecting the already-clogged lawmaking process to be ready to amend a law every time a new compound ends up in water, is extremely unrealistic.
That's... exactly what this ruling correctly determined; that the power must remain with the people and their representatives.
Institutional zealotry is rarely a good thing.
In France, the law would be typically written as "The water must be safe to drink. The list of banned chemicals and safety thresholds is to be set by the Supreme Court of the Administrative Branch" (Conseil d’Etat). That way you still do have a judicial oversight of agencies, but with a judicial branch which has more adapted procedures to those issues (for example you don’t have to wait for a case to go all the way up to the Supreme Court to adjudicate — it is literally part of the job of the Supreme Court of the Administrative Branch to be proactive in those matters).
If they are not happy with how an agency is acting - they have the clear and obvious ability to amend the laws that govern that agency.
Their failure to do so is not arbitrary - it is agreement and faith in the agency they have empowered to act on their behalf.
The courts are now compelling action. Why?
If I am satisfied with how my employees are behaving, I don't chastise them or correct them over minor ambiguities - I leave them alone to continue behaving in a manner I approve of. If I am unsatisfied - I take action to correct the reason I am unsatisfied.
The courts seem to have turned this common sense on its head.
The lawmaking body of the land is congress, and they are comprised of a fair representation of the people. That is by design.
Changing the law requires a majority in the House and Senate and either a veto-proof majority or the signature of the President. It's not supposed to require only the President.
But now you have a problem. What happens if the law that actually passed is ambiguous? Who resolves the ambiguity?
The courts get the first pass. Then if there is consensus they got it wrong, Congress can change the law. If there isn't consensus they got it wrong then the elected branches are deadlocked and the courts act as the tiebreaker until they reach consensus.
Making the President the tiebreaker is giving way too much power to one person.
Let's be clear: a lot of these rules are policy decisions. Congress and the Executive are supposed to hash out policy, seeing as they're the only branches controlled by elections. The Judiciary shouldn't have a say. But, without Chevron Deference, policy can be decided by a private party and a judge. That's a terrible way to guide a country.