I'd argue that the last season of B5 was absolute crap. He went out of his way to wrap everything up (quite well, I might add) in the 4th season that he had nothing to do in the 5th, and people started acting well out of character for no reason. (Lennier comes to mind. I'm still pissed off about that one.)

I'd personally argue that the plague on Earth is somewhat irrelevant to the story. I'd also argue that JMS is a great plotter (even if he did lift large portions from The Lord of the Rings), but a lousy scripter. (As evidence, I submit every standalone B5 "movie".) And he'd present that story as a standalone plot, and it would suck. So that may influence my opinion of its relevance.