Why the last story in Denis Johnson's last collection doesn't work at what that reveals

Sadly, Denis Johnson's final story, Doppelganger, Poltergeist, the last entry in his last book, The Largesse of the Sea Maidess (yes, he was not great on titles), is the weakest entry in this fine collection, but understanding why this final story doesn't quite succeed actually shows us why at his best Johnson was inimitable. The D,P story tells of a poet/professor, the narrator, who may bear some resemblance to DJ, and his lifelong friendship with a student: we first meet the student, Mark, in a seminar the narrator is leading at Columbia; the narrator in complete frustration at the academic world goes off the rail during a seminar, lectures or harangues for some time about of all things Elvis Presley and how he was ruined by his manager, the Colonel; the students are puzzled, except for Mark, who is clearly the seminar star. Over time, narrator and Mark see each other on occasion and stay in touch, and most of their conversations have to do with a weird and complex conspiracy theory that Mark harbors regarding Elvis - belief that E was murdered (by the Colonel) and replaced by his thought-to-be-dead twin brother (apparently there are people who believe this story). The problem w/ this narrative in my view is that DJ spends an incredible amount of time letting Mark explain his various theories - all the while the narrator worries, over decades, about Mark's mental stability. By the end, we don't really care about either man especially nor do we in any way find credibility in Mark's theory (nor are we meant to). Contrast this w/ the other 4 stories in this collection: What makes Johnson at his best so great is that he describes a world - often one of drug addiction, despair, imprisonment, and ruined lives - that is nothing like the world that we, that is most of his readers, have ever experienced and we believe in it completely. This final story is the opposite: a world that most of us (particularly other writers and teachers) have experienced (the brilliant troubled student acolyte) yet we don't really believe it (i.e., the facts and particulars of this story) at all.