Lawyers have a saying: Hard cases make for bad law. In other words, the more unusual and extreme the circumstances, the more damaging the precedent it sets for all the other cases to follow.
Barr and the others should have realized that Trump is as hard a case as there is.
You were never going to preserve the integrity of the presidency by defending the least honest, most reckless example of presidential leadership we’ve ever seen. You can’t shore up the institutions of government by enabling a man who wants to dismantle them.
Trump isn’t the case you seize on to uphold precedent. He’s the case you call an outlier and toss aside.