The thing with FFXIV is that it's a character-driven story with a lot of characters; the end of a story is ultimately the culmination of a lot of people's individual journeys. As a result, it becomes really hard to pose a 'what-if' that doesn't end worse, because we tend to be faced with only problems this particular lineup of characters can solve.
Stopping Sphene is ultimately the culmination of multiple character journeys: most prominently Krile, Cahciua (and therefore Erenville by extension), and a couple we never mentioned, like Gulool Ja and Wuk Lamat; the fact that the Queen Eternal is even beatable when we do get there is because Wuk Lamat connects with Sphene enough to get her to actually stop. That does mean that the problems in front of us are often very specific problems, that can't just be dismissed or easily done by someone else, because Character X doing Thing Y is an important part of their journey.