Some time passed, the event did not happen. Same person said: ok, 50%, either happens or not.
Some time passed, the event did not happen. Same person said: all my previous predictions were correct, now let's apply Bayesian update: (1-0.9) * 0.5 / 0.5 = 0.1, so now posterior belief that the event is likely to happen drops from 50% to 10%.
Second, it completely ignores any debate over effectiveness or side effects. It could well be that fluoride in water is great for teeth but bad for brains. The objections to fluoride in water I’ve seen are more along those lines. Im not clear the validity of those claims but for example anti fluoride advocates don’t typically object to chlorine in water to kill germs. That seems the core issue- without bias from stakeholders, is the benefit of fluoride proven and the risks disproven? It’s hard to answer because a study needs to span many years and exclude many variables.
And in general I think that is what needs to happen with these type debates. Take them _out_ of the sphere of charged political opinion and focus on getting to the objective truth of risks and benefits, then be transparent. People can handle “here are the known pros and cons and what we think that means” over “there are only pros and no cons and if you disagree you hate poor people”
This is strongest in the "Sleepytime" episode which is based on the "Jupiter" movement of Holst's "The Planets" . . . honestly I have to skip this episode when it comes up because it makes me tear up so much, and most parents I know who also watch the show have similar reactions. "Sleepytime" is really art.