We should probably keep doing studies to check how dentists think we should brush our teeth. We just shouldn't have dentists do a study on a variable that directly harms their income as we would already know the results they would give before the study is even done.
If you wanted a study on that variable, you'd likely need to at least get input from someone who says going to dentists is a bad idea - but at that point, you're just going to have a shouting match and a statistics measuring contest between the dentists and the guy who doesn't think anybody should see dentists. Your results would likely be contradictory and fairly useless.
We haven't yet found a way to do such studies in a useful manner, and it's an ongoing problem in politics that has yet to have a solution. And may never have a solution. And it's very important to be able to determine when such study variables align with the financial and ideological requirements of the researcher.
If you can tell without doubt what the result of a study by a certain group will be before the study takes place, it's usually a red flag.