Let me propose another way to look at this (I have no idea what to think of Mueller to be honest other than he has a lot to answer for in his past).
Anyways, I digress. If he was "our guy" those millions already thought he was guilty before anyone(clueless anti-trumpers) heard of Mueller and if Mueller finds no evidence of wrongdoing at least it gives Trump more credibility when he goes on the offensive after he is found not guilty.