You can say that about anything. If Gohan wasn't there, Goku could become a SSJ2 and defeated Cell.
@ahill1 Exactly, everything is plot. It's dumb to argue that if a character didn't do something, something else wouldn't have happened because ultimately, everything is decided at the author's whim. It's just fiction. In the end, everything comes down to "the author felt like it" and thus, all of it is arbitrary.
But as far as the franchise is concerned, some things are less arbitrary than others. Because
Dragon Ball is primarily supposed to be a fighting series, the fighting matters the most. Without fighting, the franchise doesn't make money, which is all it's meant to do at the end of the day. That's not to say that Toriyama and his company's desire for the franchise to make money isn't inherently arbitrary, because everything in life is arguably pointless, but if not for the desire to make money, the franchise wouldn't exist. Maybe nothing in life matters, but if the franchise is intended to be successful, then it needs to make money. If that's a goal, then the stuff that makes money is less arbitrary than the stuff that doesn't.
Toriyama didn't need to have Bulma bring Goku out of the mountains when someone else could have done it in the story, just as he didn't need to have Gohan beat Cell when someone else could have done it in the story. Regardless of who beats who, though, you still need characters kicking ass. If no one's kicking ass, then the franchise doesn't make any money. If it's not making any money, it's not successful and thus, it has failed in its goal. Bulma doesn't fight, so she doesn't kick any ass. That's why she's worthless in the story. All she's used for is being the butt of countless perverted gags. Dragon Ball isn't meant to be a sex comedy, though (which is why there are ultimately very few sex gags throughout the whole story), nor is it meant to be about scientists inventing things (why is why all the inventions are made off-screen); it's just meant to be about fighters kicking ass.
You could suggest that Bulma could have been a fighter who kicked ass, but she wasn't. If you made her a fighter who kicked ass, then you'd be changing what the viewers see, just as you'd be changing what the viewers see if you make Goku the one who kills Cell instead of Gohan. The viewers didn't see that, so it's a moot point. Whereas if you had someone else bring Goku out of the mountains, make the Dragon Radar, make the Time Machine, etc you're not changing much of what the viewers see. Those things are minuscule in the story, because they don't pertain to fighting, the only thing that matters. No one cares about the other shit except nerds.
tl;dr Everything in Dragon Ball is arbitrary but especially the stuff that doesn't have to do with fighting because only the fighting is necessary for the franchise to be successful. Bulma doesn't fight so she's useless. Game Set and Match fammy fam