You can have infidelity in an HEA fic too theoretically, the couple work through it and move on to their real HEA. I feel like people keep setting up this false dichotomy between fluff romance and darker literary plot fic, that isn't really an issue.

I've always liked happy ending romances, and even I don't find the fic these days very satisfying. There's just a general lack of the characters doing things and making choices and in the end they are so passive and the storylines are so thin, the authors can't even include an ending. The stories just stop before the denouement.

(Not specifically a reply just to you, but something I've been thinking about while reading this thread.)