I somewhat disagree with this. Obviously we won't ever know for sure what would happen if Aerith lived (unless she somehow does in the Remake), but if we're going by narrative intent and logic, then Cloud and Tifa would still end up together by the end of the story. In fact, Aerith living would actually improve Cloud and Tifa's relationship, because then they'd have no guilt to deal with post-game.
Say Aerith did live, but everything else in the game plays out in the same way. At some point, Sephiroth still breaks Cloud, Cloud falls into the Lifestream, and winds up catatonic in Mideel. Tifa is still the one to stay by his side (maybe Aerith does too, but she has more planetary issues to take care of, so I doubt it) and falls into the Lifestream with him. Tifa puts Cloud's mind back together in the most intimate sequence in the story, and Cloud realizes he left Nibelheim to impress Tifa and that he's liked her since childhood.
Post-Lifestream sequence, I don’t see a route for Cloud and Aerith to develop a relationship. What, Cloud's going to remember his past, realize Tifa reciprocates his feelings, and then he's going to tell her, "Well, I've loved you for a long time and left town to impress you, but I've actually realized that we're better off being friends and I love Aerith now so bye."
If we're being indulgent, maybe they date for a bit, but at that point, Cloud and Tifa’s bond is so strong and entangled that — narratively speaking — it wouldn’t make sense for Aerith and Cloud to be together. Aerith wouldn’t be able to breach that same level of understanding that Cloud and Tifa have with each other; there would always be a barrier. When Cloud’s memories return, so does his old flame for Tifa. There's no "moving on" with Tifa, because Tifa was always the one Cloud loved in the first place — he just couldn't remember that for a short 2 months.
This is all under the assumption that the writers of the story understand basic story structure and character development, which they definitely do. The Lifestream sequence kills any ambiguity in my opinion — everything after that is just gravy.