> I can think of at least two operas with different plots without even trying.

Obviously it was a generalization, but it's one that holds in a surprisingly high percentage of cases, and that's all that matters.

> The OAS doesn't take into account 'light opera', which is full of Humorous Misunderstandings and
> ends with everybody having a party.

That, I believe, is called "operetta," which is a whole different ball game, not just storywise, but musically, as well. Correct me if I'm wrong, though, as I may well be. At any rate, what Gilbert and Sullivan wrote were operettas, not operas.