"To Justify Something Is To Diminish It?"
Yes, of course.

Fish's arguments, and their weaknesses, are pretty clear. It's easy to understand if you know the antecedents -and isn't that part of Holbo's job description to know them?- But Holbo is "vexed."
The only example he can come up with to parallel Fish's argument is etiquette. Etiquette comes at the end of the line. It's the argument for manners when they're no longer founded on anything but themselves. Mozart's music is founded on and is considered the high point of a tradition. T.S. Eliot and Kentucky Bluegrass Banjo players are inheritors of tradition. Sonata form, lyric poetry, meter and rhyme scheme, and on and on. It's not that Holbo wants to make an argument against these things and their role in the academy, or even that he's accusing Fish of overreaching, it's that he arguing from an ignorance that they ever had a role. That's just bizarre.
I quoted Eliot:" He had a mind so fine that no idea could violate it"
You may not agree with the sentiment but ignorance is no excuse.

As I said in a comment, it struck me for the first time how the contemporary culture these people are most attracted to is basically Pre-Raphaelite: proto-fascist and borderline kitsch; Randian, life as art/ art as life; life/art ordered by intention.