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Just Here for Godzilla with the "Godzilla" in this case being the B-plot?
Agree that it's probably a variation of Just Here for Godzilla, though that trope generally means people are only there for a specific scene or character that makes a big splash, whereas a focus on the B-Plot implies a bit more depth in the fan's interest.
I'd say it's related to Periphery Demographic, where say, a movie that was supposed to be an exciting sci-fi film performed poorly with sci-fi fans because the effects were crappy and the generic 'save the world' plot was weak, but it picked up a sizable chunk of attention from fans of romance, because the love story (the B-plot) was really good.
Edited by SpocktorWho
Yeah, it seems like we'd already have a trope for this, but I can't think a specific trope. I guess it could be Come for the X, Stay for the Y, with the Y representing the B-plot. It could be They Wasted a Perfectly Good Plot, where the plot the audience is more interested in isn't developed.
Edit: It could also fall under Ensemble Dark Horse if the reason the audience likes the subplot more because of a certain character that appears in it. To me, there seems to be a lot of tropes that this could fall under, but not one trope that specifically describes it.
Edited by ShawnRi
When the audience finds the B-plot of an episode more interesting than the main plot. It seems like we would have a trope for this, but I can't seem to find it.