Not to refute your n=1, but reviews are a safer way of establishing whether something actually works. Eg here, from 2015:
> There is insufficient information to determine whether initiation of a water fluoridation programme results in a change in disparities in caries across socioeconomic status (SES) levels.
> No studies that aimed to determine the effectiveness of water fluoridation for preventing caries in adults met the review's inclusion criteria.
> Over 97% of the studies were at high risk of bias and there was substantial between‐study variation
> There is insufficient information to determine whether initiation of a water fluoridation programme results in a change in disparities in caries across socioeconomic status (SES) levels.
> No studies that aimed to determine the effectiveness of water fluoridation for preventing caries in adults met the review's inclusion criteria.
> Over 97% of the studies were at high risk of bias and there was substantial between‐study variation
From here: https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD...
TLDR: The existing research is low quality.