To quote Schmendrick: "There are no happy endings, because nothing ends." Trouble in your relationship means that it's time for you to grow some more. Being committed to growing together is what partnership is, so it can't exist without trouble. So when I see Cloud and Tifa having trouble, I see them growing together -- it's my favorite thing about AC. In DoC, the story didn't touch on them, probably because there was nothing to report. Happy times don't make compelling drama. So, I think DoC (so far the farthest in the timeline we've gotten... within their lifespans) is the happiest ending we could ask for. Nothing to report.