If you watched Sailor Moon Cosmos movie, you would know Sailor Cosmos. She comes to the past as Chibi Chibi to confront her younger self and fix things, due to Chaos's reappearance in the future. Point is, if Sailor Cosmos time travels to change what her past self did, then Usagi's fate is already chosen and we're certain that Chaos will come in the future. Maybe if Cosmos never came, Usagi would still have free will and could make her own decisions. Why would Cosmos come if common sense would tell her that the future simply can't be changed? There was no point in coming to the past, Sailor Moon would have sacrificed herself anyway. Also, I thought that if two of the same people were in the same timeline, then it would tear the fabric of reality (As proven in SM Crystal S2). How does this work if Usagi and Cosmos are in the same timeline? Plus, if Sailor Cosmos is the "greatest Sailor warrior," then why did she need to go back to the past rather than use her powers to actually fight? We know that she's stronger than Eternal Sailor Moon, maybe even Queen Serenity herself. Cosmos also has the other Sailor Guardians AND Chibiusa AND King Endymion AND Chibiusa's Sailor Guardians to help her, so why time travel when you've got an entire team of fighters? It kinda defeats the purpose of hope and destiny in SM, because even though we receive a happy ending where Usagi and Mamoru get married and have Chibiusa, now we know that there will be another war in the future.
I have so many more questions about this that will probably never be answered. If you guys have more theories, please drop them in the comments. Like, why is Sailor Cosmos' sailor outfit look drastically different from Sailor Moon's? I guess I'll never know :')
