It took that long to repeal because it was an amendment (and thus needed another amendment to repeal it). The rest of your post basically says people drank less therefore less people drank (it's obvious that if less people drink, less people will have alcohol problems). I'm not sure if the academic consensus is that it was "mixed," most top search results seem to agree that it was kind of a failure[1][2], but to be fair I never studied it in depth and Google can be biased.

[1] https://www.cato.org/policy-analysis/alcohol-prohibition-was...

[2] https://www.loc.gov/classroom-materials/united-states-histor...