Gohan's swift descent from Earth's strongest fighter is a hot-button topic amongst Dragon Ball fans, with many considering him to be a major disappointment. It's time to change the narrative around the half-Saiyan's choice to no longer walk the path of a warrior. Considering everything Gohan contributed to the series as a kid, the young man has more than paid his dues as a fighter.
It's important to remember that Gohan was only five years old when he had his neck broken on an alien planet. He was only 11 years old when his own father gave Cell a Senzu Bean, who then proceeded to crush Gohan's ribs in a bear hug. Gohan spent most of his formative years watching people die. Goku, Yamcha, Chiaotzu, Tien, Piccolo, Dende, Vegeta, Krillin, Android 16, Trunks and Goku again, all die in front of him at some point -- and those are just the people who died in front of him that he actually cared about.
It's hard to blame Gohan for wanting a normal life. His hidden potential saved the Z Warriors on several occasions, but that doesn't mean he ever actually enjoyed fighting. It's so difficult to tap into that power because, as a half-Saiyan, despite having greater potential than a full-blooded Saiyan, he lacks the necessary thirst for battle and drive to get stronger. Even when he is forced back into battle for the Tournament of Power, he states that he chooses not to rely on his Saiyan heritage, but rather his humanity.
Maybe if Goku were a better father, Gohan wouldn't be racing to the nearest book whenever a battle for the fate of the world has ended. Inattentive to say the least, Goku has spent the vast majority of Gohan's life either away training or dead (sometimes both). Even after Goku survived the destruction of Namek, he opted to remain in space because of reasons, rather than return home to his son who spent almost two years looking up at the sky waiting for him.
As a child with two extremely overbearing parents who force their desires on him, it's no surprise he gravitated toward the parent who was actually around. Gohan has seen what an obsession with getting stronger does to your personal relationships. Unwilling to make the same mistakes as Goku, and rightfully so, he dedicates himself wholly to his family. While his father will fight for anything, Gohan will only fight for his loved ones.
Too often Gohan's passive nature is mistaken for weakness. When the Tournament of Power was announced, Gohan agreed it was time to suit back up, so to speak. Remarkably, he quickly re-achieved his Ultimate form and then some. He was directly responsible for the destruction of multiple universes by eliminating their final members in the Tournament of Power. His performance in the tournament reinforces the notion that he chooses not to be a warrior, not that he's incapable of it.
Gohan has the life he truly wants and it's certainly more than his father has. It's not that Goku doesn't age, it's that he hasn't aged yet. When he finally does grow too old to fight, all he will have are memories and a family that he's grown apart from. It is to the benefit of his character growth that Gohan can recognize the shortcomings of someone he loves and work to avoid the same behavior because he knows how that behavior affects others.
All that said, these days in Dragon Ball Super, Gohan is simply not needed. Even though they couldn't have been less helpful in the Cell Saga when they forced Gohan to clean up their mess, Goku and Vegeta are the heroes now. They've become so powerful that Gohan could never catch up, and that's fine. Gohan is living his best life, which is a peaceful life. And if Goku ever needs his help, Gohan will be there to give the enemy a Senzu Bean.
