"In Switzerland 85% of domestic salt consumed is fluoridated and 67% in Germany. Salt fluoridation schemes are reaching more than one hundred million in Mexico, Colombia, Peru and Cuba. The cost of salt fluoridation is very low, within 0.02 and 0.05 € per year and capita. Children and adults of the low socio-economic strata tend to have substantially more untreated caries than higher strata. Salt fluoridation is by far the cheapest method for improving oral health. "

https://pubmed.ncbi.nlm.nih.gov/24308394/

(Sea salt and Kosher salt are the salts that aren't fluoridated and iodated in those countries, fancy, more expensive salts, regular table salt- and the salt added in commercial/restaurants has both.)

So sure, you don't need to fluoridate the water, if you fluoridate the salt instead. But you have to do it some way or another. And the US and Canada doesn't, at present fluoridate the salt because we have it in the water. Remove it from some people's water but don't add it to the salt because everyone else has it in the water? Bad combination.

“Dual fortification” is mandatory in Mexico.