Romeo and Juliet might voice an objection here. It's certainly a common trope in the entire anime/manga/LN sphere, but as an overarching genre of storytelling I think this expectation would not be grounded in reality.
I mean that a happy ending is literally listed as a requirement by the Romance Writers of America (I'm aware we're talking about Japanese media, but it's more or less the same there):
Two basic elements comprise every romance novel: a central love story and an emotionally satisfying and optimistic ending.
Romeo and Juliet is a tragedy, not a romance. That's why I said it depends on how you define the word.
Yeah, I feel like this is the most trivial caveat ever... have you ever been in a relationship? Relationships are never "happily ever after with zero tension for the rest of your life". If anything, it's the opposite. Merely depicting a relationship provides boundless opportunities for conflict and drama.
Of course, I never said it was impossible, just that it's more difficult to come up with conflict outside the standard romance template. In particular, it's very difficult to write conflict between two characters who are basically good or at least who the audience are expected to sympathize with; this is why most fictional antagonists are one-dimensionally evil, and why most obstacles in romance are external (misunderstandings, evil parents, love triangle, whatever).
I would also venture to guess that a large subset of the romance audience wants to read about an "ideal" relationship they can self-insert into, and bluntly depicting the difficulties that arise from (most/all) real-world relationships ruins that wish fulfillment aspect.
To be clear, I do agree with you that I'd like to see more work that seriously tackles realistic disagreements and conflicts that arise between two (or more!) people in an established relationship. I'm just theorizing as to why those works aren't particularly common.
Hmm, I don't agree, though? I think it's done a perfectly fine job of continuing the story, with stuff like tension about how much time they spend together, their plans for the future, coming out to their friends. Mind you, it's a fluffy, low-drama story where all of the issues are generally resolved quickly and happily, but it was also like that before they started dating, so I don't think that's really related to the topic at hand.
I confess I didn't love the manga before they got together either (not because it's fluffy and low-drama, the characters just aren't particularly compelling to me), so that's a fair point.