I agree that just dropping a seemingly safe belief should take effort. I have no problem challenging them on a regular basis, though. Especially with more than just arguments and thought experiments. More observational studies can almost certainly only be a good thing.
Also, that UV damages DNA and causes cancer isn't simply a "proposed" mechanism, it's been tested many times.
I would say pure "observation" per se has little value -- for it to be scientifically valid it needs to seek explanations.
The mechanism isn't really at question. But just as sun exposure can kill grass, so to can lack of sun. We know that humans don't do photosynthesis, but it seems reasonable to ask if we do get other benefits.
Note. Is reasonable to ask and study. Is not reasonable to just assume the conclusion.