We've been putting fluoride in water for a long time, and there are multiple studies that have looking in to it, I don't know why you think there aren't?
I guess it's arguable whether better dental health, plus cancer, could be considered a benefit. Would heavily depend on the rates of cancer or other harms vs. the rates of improved dental health. Certainly without thinking too deeply about it, childhood cavities would seem vastly preferable to nearly any sort of cancer unless the risk is minuscule.
Assuming you are not being disingenuous: there is a trade off with a lot of uncertainty - other things are also implicated in stomach cancer, and at the same time untreated dental disease causes other health problems too.
But I prefer to rely on brushing my own teeth and avoiding certain foods to maintain my own health, rather than have a medication added to my water supply because the guy next door does not take care of his health.
There are studies for children, but from what I’ve seen there’s no advantage for adults/permanent teeth. I don’t really think it’s worth potential harm for baby teeth.
It's harder to do studies that seperate out adults who didn't have fluoridated water as children, but for example from https://pubmed.ncbi.nlm.nih.gov/23456704/ "In this nationally representative sample of Australian adults, caries-preventive effects of water fluoridation were at least as great in adults born before widespread implementation of fluoridation as after widespread implementation of fluoridation."
So if you didn't have fluoridated water as a kid, having it as an adult still provides a benefit. This particular study doesn't say anything about having it as a kid and then stopping, but again, it's hard to find significant populations that have experienced that.