While Trunks has literally crossed the time stream–twice–to ensure the survival of his world, he still fails to come out on top as even after celebrating his greatest victory, Trunks becomes responsible for Dragon Ball Super’s biggest mistake.
Fans were first introduced to Future Trunks in Dragon Ball Z Chapter 136 “The Comic of King Cold” by Akira Toriyama. That chapter launched the beginning of the “Android Saga” as Trunks only traveled back in time to keep the Androids from rising up and decimating his world. After he helped defeat the Androids in the past, Trunks returned to his future only to find that nothing had changed, realizing that the changes he made in the past had no impact on his future. Remembering that piece of information, Trunks traveled to the past once again in Dragon Ball Super, only this time he went to bring Goku and Vegeta to the future with him to defeat arguably the most powerful villain in Dragon Ball history, Zamasu.
In Dragon Ball Super Chapter 26 by Akira Toriyama and Toyotarou, Trunks celebrates his greatest victory as he and his allies are successful in defeating Zamasu after recruiting the help of the Omni-King Zeno. Afterwards, all of the surviving fighters go back to the main Dragon Ball timeline as Zeno destroyed the entirety of Future Trunks’ universe when he obliterated Zamasu, making it impossible for Trunks to return home. However, Lord Beerus and Whis offer to bring Trunks to an alternate timeline nearly identical to his home but without the threat of Zamasu. The only catch is that there will already be another Trunks in that world roughly his age, but Trunks doesn’t see a problem with that and takes Beerus up on his offer.
Trunks going off to another timeline where another Future Trunks already exists was a huge mistake on the part of Dragon Ball Super. Future Trunks has been a fan-favorite character since his debut, but he was never a main member of the team because his destiny was always to return to his own timeline and save his world. In this storyline, however, Trunks' home world is destroyed, giving him every excuse to simply stay in the present day and officially become a member of the Z Fighters from that point onward. Plus, living in a world where another version of oneself exists as a young child is way less weird than living with an exact copy, and younger Trunks already thinks of Future Trunks as his big brother, so that situation would have actually worked out perfectly.
Future Trunks could have still had his own identity if he stayed in the main timeline separate from his younger self. Plus, he would have been a huge asset in the Tournament of Power, so his staying would not only have been beneficial to fans of Future Trunks who want to see him finally be made an official Z Fighter, but also to the Z Fighters themselves. Overall, the main timeline would have been a much better home for Trunks as opposed to a universe where he has to live with an exact copy of himself, proving that Dragon Ball Super’s biggest mistake came from Trunks’ greatest victory.