Dragon Ball and the Marvel Cinematic Universe both share the same time travel problem as shown in both the Trunks Saga and Avengers: Endgame!
Both Dragon Ball and the Marvel Cinematic Universe have proven that time travel is possible in their respective universes. Dragon Ball first introduced the idea when Future Trunks came back in time to the present day and teamed up with Goku and the other Z-Fighters to stop two powerful villains with the hope of saving his future. The Earth’s Mightiest Heroes of the MCU had to meddle with the sacred timeline in order to undo the devastation Thanos brought to the universe when he used the Infinity Stones to erase half of all life throughout the cosmos. While both groups were eventually successful in their missions, they both faced a similar problem, one that otherwise would have made their goals far easier to attain.
In the sixth saga of the Dragon Ball Z series, “Trunks Saga” by Akria Toriyama, Future Trunks goes back in time after his future is ravaged by Androids 17 and 18. In his travels, he saves the life of Goku, who would have otherwise died of a heart disease, and works with the Saiyan and the rest of the group including his father, Vegeta, to stop the Androids before they destroy the world as they know it. After the Z-Fighters witness the rise of Cell and Goku’s sacrifice to end his reign of terror, Trunks goes back to the future, hoping to have all the destruction he once knew undone and to see the reward of his victory. Unfortunately, the future Trunks returns to is unaltered, even after the heroic changes he helped make to the past.