The meaningfull context here was "good diet". If middle distance runners ate for middle distance running would they need to take beta alanine? E.g. ate more organ meat which contains high levels of beta alanine.
How much organ meat do you think would be needed daily to have an optimal dosage of beta alanine? What side effects are associated with eating large quantities of organs meats regularly?
Honestly, likely not that much. From what I've found you can get recommended dosage and more from just eating the right foods. Right meaning foods rich in what you're looking for. Now in context that may be hard for some people or they just don't like those foods. So supplementing makes sense.