This is pretty much a canned response from LatteLazy. It ignores the fundamental fact that science has never “proved causation” of anything ever. All we have is lower and higher confidence correlations and that’s all we’ll ever have.

I would not call study worthless, but to establish causation one could do an experiment where elderly adults are given (1) an opportunity for a free hearing test and hearing aids if needed (2) the equivalent amount of cash. Then follow up the two groups to see if group 1 is less likely to develop dementia. My guess is that the results will not be strong.

It becomes worthless when we never take the second step.

It is worthless because why not just jump straight to step 2?

The answer of course is that that would be expensive and hard and might not prove anything. Much safer and cheaper to NOT actually test you hypothesis and you can still use it to up your publication figure and your funder can still use it to sell hearing aids etc...