Advent Children was a product of it's time, but also of it's medium. People try to gleam so much, too much, from a 100 minute movie that had to first and foremost introduce a brand new plot for a long established story - and somehow make it credible - and, secondly, have enough action scenes to make a genuine spectacle.
Nevertheless the movie tells you Cloud, Tifa and their adopted child Denzel live together as a family. Under normal circumstances, this would be about as unambiguous as the movie should have to be to get it's point across, so the film does little else to elaborate on the subject, instead moving ahead with the drama and ups and downs on personal and general levels that come with it. Just because some people believe CA's brief encounter constitutes the entirety of their characters and, as such, the entire franchise must be interpreted in light of Clerith - doesn't mean that more needs to be done to show Cloud and Tifa are together. After all, would this same group have even accepted it?
The movie delves deep into tackling Cloud's depression, a topic sadly romanticized by fans of CA. His struggles with coming to terms with the deaths of Zack/Aerith being compounded by Denzel catching geostigma, and his own helplessness as he succumbs to it also, are not some indication that he's out looking for a way to die to reunite with Aerith. Indeed, Cloud does reunite with Aerith anyway in ACC and it's as platonic as one would expect for any man seeing his best friend and his best friend's girlfriend together - he calls her mother, they joke about adopting him, he's sent on his way. All this is to say that, character relationships generally take a back seat to Cloud's own coming to terms with himself and his past, which is the key element of the film.
How many happy days has he spent with Tifa and the kids? Aye, he asks Tifa to close the bar so they can all spend time together by the end, but generally Advent Children doesn't say because it's not the thrust of the film. The plot, the action sequences, the graphical spectacle, the setting up a plot for the Compilation, these things come first and arguably define the experience. It's not ideal, but it's understandable.
That said, I will give ACC credit where it's due. It ends on a note that is suitably positive and hopeful for all. Aerith is lonely no longer, now shown explicitly being together with Zack seemingly for an eternity - which was all she ever really, truly wanted. Cloud meanwhile, finds his "Promised Land", as the 10th Anniversary Ultimania describes it, seeing out his days together with Tifa and the kids. Indeed, the Ultimania goes on to say;
"When Cloud awakens, he finds himself among his companions and children who've been freed from their deadly disease. His family is there waiting for him: Tifa, Marlene and Denzel, who asks to be cleansed of Geostigma by Cloud's hands. Surrounded by blessings, Cloud realizes this is the place he's meant to live"
It's not perfect, but you know what? This is a pretty damn good ending for the characters, all things considered.
Maybe they were right.
