You mostly need flouride when you are younger. That’s why you get flouride treatments as a kid between age 6 and 12, and not as an adult.
Its necessary to floruidate water forever because there are always new kids getting born. Its not like it changes genetics and makes kids be born with super teeth.
But, there’s no guarantee that the new children are going to drink the fluoridated water. Not everyone drinks tap water.
Plus, pediatricians prescribe supplements for young children that include fluoride, so the children could receive it that way. There’s also fluoride toothpaste and, as you mentioned, fluoride treatments at the dentist’s office. And fluoridated water is sold at the grocery stores. There are so many options.
Plus, if only children need it, then it’s not necessary to put it into the water supply for everyone to drink.
For the record, I’m not saying that fluoride is good or bad. I use it myself. It should be a matter of personal choice, imho.