Hardening the tooth enamel by fluoride is exactly like an anti-corrosive treatment of a steel surface.
If someone would propose that instead of applying the anti-rust layer on the external steel surface, a hole should be made in the steel, then the anti-corrosive coating should be put inside, then the hole should be plugged, enclosing inside the anti-corrosive material, and after all this they would expect that the external untreated steel surface will not rust, they would be laughed at. But this is equivalent with the theory that one should drink fluoridated water to harden the tooth enamel.
[1] Article/study discussing Calgary vs Edmonton in Canada, linked elsewhere in the comments here that has this snippet:
What’s happened since 2012? A study found that within a couple of years of ending fluoridation, decay in children’s baby teeth had risen by 65 per cent compared to 2005, while the increase was only 14 per cent in Edmonton, which continues to fluoridate its water.
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I won't argue the fact that there may indeed be problems with ingestion, and additionally that contact to the teeth is the only way for the fluoridated water to help, but I'm not convinced that existing water fluoridation isn't having a positive effect on tooth decay. Whether it's worth other potential health trade-offs is certainly a valid concern.