This is usually just used as a sneak attack on someone's else's suggestion, a way to call it unrealistic without actually making a case that it's unrealistic. R

This is usually just used as a sneak attack on someone's else's suggestion, a way to call it unrealistic without actually making a case that it's unrealistic. Rest assured, the people who tell you not to let the perfect be the enemy of the good do not think that what you suggested is either perfect or good, they just want you to shut up. The "fallacy" in this vein that I see is when after Bob suggests idea A to solve problem X, Mary says that idea A shouldn't be done because idea B is better for problem X, but Mary also doesn't support idea B. Mary actually supports problem X, but if she admitted that, she would lose her influence on the reaction to problem X.