There have been a lot of changes in the Dragon Ball franchise, with Dragon Ball Z being perhaps the biggest example of this. These chapters in the franchise revealed several hidden truths concerning major characters, namely Goku and Piccolo. The former was at one point almost replaced as the series' protagonist, but this change was ultimately undone.
The reason for this is hotly disputed, especially since it only becomes "clear" as Dragon Ball Z starts coming to an end. Gohan was meant to take over much earlier, but even when he was ready for the limelight, Goku was the one who saved the day. In the end, Goku remained the protagonist of the franchise because he was better suited for the role.
Gohan Was Meant to Be the Hero of Dragon Ball Z
Gohan is Goku and Chi-Chi's Son
Gohan is introduced right at the beginning of the Dragon Ball Z anime and its corresponding chapters in the Dragon Ball manga. Shown as the young son of Goku and his wife Chi-Chi, the character was a lot different from his dad. Whereas Goku was raised by his adoptive grandfather (also named Gohan) in the wilderness with a big sense of adventure but little regard for civilization or manners, Gohan is an erudite and somewhat shy boy who's made to focus on his schoolwork by his mother.
Both of their worlds are suddenly thrown upside down with the arrival of Raditz, Goku's evil brother, and Gohan's uncle. Revealing the family's alien nature, Raditz requires Goku to sacrifice himself in order to be killed. Thereafter, Piccolo (once a fierce enemy of Goku) begins training Gohan for the arrival of more Saiyans who are headed to Earth. This is one of several examples where it seemed as if Gohan was going to become the protagonist.
Of course, Goku was quickly revived by the Dragon Balls, but there were still other examples of Gohan taking the lead. He remained an important and potent young fighter throughout the Frieza Saga and even non-canon storylines such as the Garlic, Jr. Saga. Easily his biggest claim to fame, however, was killing Cell after the villain had done the same to his father. Following this death, Goku had resigned himself to being dead for a while, hoping to leave Earth in the hands of a new generation of protectors.
For a while, this actually remained the status quo, with Gohan going to a normal high school while still training and also fighting crime as the "Great Saiyaman." This next major story arc builds into the Majin Buu Saga, which was the final part of Dragon Ball Z. More than ever, Gohan was given a level of importance befitting a main character. For instance, he underwent special training, earning a new form called "Mystic Gohan" and becoming more powerful than ever.
Even Majin Buu callously hypes his opponent up as the supposed "savior" meant to defeat him, though this sadly wasn't Gohan's actual fate. Instead, the villain defeats him and Gotenks (the fused form of Gohan's younger brother Goten and Trunks, the son of Vegeta and Bulma), with Goku and Vegeta being the ones who end up beating Buu. By the end of Dragon Ball Z, Gohan is just another spectator watching his dad save the world over and over again, despite that role seemingly being prepped for himself.
After all, Goku had eschewed killing Buu in an earlier, weaker form so that Gohan or someone else could do the job. This plot contrivance was definitely a mistake down the road, but it also speaks to how Gohan simply wasn't up to the task of being the main character.
Toriyama Didn't Want Gohan to Replace Goku
Goku Became Very Popular With Fans
Franchise creator Akira Toriyama had high hopes for Gohan and wanted to start things fresh at the beginning of what became known as Dragon Ball Z. This is why Goku was killed off so early in that series' storyline in order to make way for the next generation of heroes. There was even an outright passing of the torch at the conclusion of the Cell Games, with Goku's spirit being beside Gohan as he kills Cell.
Even the fact that Gohan went to school and began to lead a much different life allowed for new possibilities in the story that focusing on Goku never would. Videl was his equivalent to Chi-Chi, though Hercule/Mr. Satan wasn't at all like the Ox King. It was all a much different status quo that felt fresh after so much of the series had simply devolved into showcasing how "special" Goku is compared to everyone else in the Dragon Ball franchise.
The reason why Toriyama reneged on making Gohan the protagonist was due to his simply not feeling that he was right for the role. It's a strange justification given all the narrative work up to that point that had gone into building Gohan up as the new main character, but it's also believed by many that Gohan (and the series as a whole) had noticeably lost popularity once the young Saiyan/human hybrid became the focus of the story.
His balancing his battles with more mundane aspects of life such as school and a normal (compared to that of his parents) romance with Videl were simply too far removed from what fans and even Toriyama himself had become used to from the series. Thus, the mangaka abruptly dropping the development and having Goku come back as the hero over halfway into the final arc made sense, despite how jarring it was.
Dragon Ball Z Had a Lot of Status Quo Changes
The Franchise Started Off More Comedic Before Becoming a Galactic Battle
As noted, Dragon Ball Z already had a lot of status quo changes throughout its story. This began with the revelation that Goku (and by extension, his son) was an alien and not simply some weird mutant with a monkey tail. A similar concept was then introduced concerning Kami and Piccolo. These two green-skinned characters were treated as being demons from Earth in the original Dragon Ball, but Dragon Ball Z revealed that they were also aliens and part of a species that hailed from the planet Namek.
Likewise, the people of that world were being besieged by the same tyrant warlord that had essentially enslaved the Saiyans before almost entirely wiping them out years before. That's without even mentioning the tonal changes made to the story and its scope. In the transition from Dragon Ball to Dragon Ball Z, the series went from an adventure comedy with fantasy and loose science fiction elements to an all-out brawl where sci-fi and fight scenes took supreme precedence.
Likewise, characters who had once been powerful and prominent in Dragon Ball were now increasingly irrelevant due to the growing gap in power. This was especially the case for mere humans such as Krillin and Yamcha, though even Piccolo was glorified window dressing by the series' end. Amid all these changes, the one recurring element from the original series really was Goku.
He was the one that fans had grown up watching and reading, so in many ways, he was synonymous with Dragon Ball to a fault. It was impossible to truly remove him from the equation, which is why replacing him with anyone (including a son who looked a lot like him) was always going to be a hard sell. Since then, Gohan has mostly remained a side character in the series, with both the non-canon Dragon Ball GT and the official sequel series Dragon Ball Super showcasing him as focusing more on his studies and business pursuits. In a way, this does validate Toriyama's feelings that Gohan would never have been able to replace Goku, though the attempts to actually do so were definitely feeble.
