>>263988612This is because Toriyama used those characters only as a way to highlight how the new enemies from each arc were strong. For example, before the Saiyan saga, Yamcha was still considered a good fighter, at least good enough in the sense that he could be of good help against the saiyans. Him dying to a saibaman set the tone and showed how dangerous the enemies were this time. Tenshinhan also was a great fighter and by that point he was probably the 3rd best fighter behind Goku and Piccolo, yet he couldn't defeat Nappa. All of those events happened because the public needed to be hyped up about Goku arriving, because he was the only chance, the strongest of them all. If other characters had been strong enough to at least annoy Vegeta a little bit, the fight against Goku would have been less spectacular, and thus less memorable. Toriyama had to sacrifice the characters he deemed not strong enough to keep up.
Before the saiyan saga, Goku powered up with training, Korin, the "holy water" etc, all of which were accessible to other humans. By the point the alien talk came in, it was basically admitted that Goku was so exceptional because he was a saiyan. This is why even after visiting King Kai, the other humans have never been able to use techniques like Kaioken, they were limited by their own nature. They had reached their full potential because there were no more powerups in store for them, no human version of the super saiyan transformation, no more ways to get back to Goku's level. Every arc, the enemies became exponentially stronger than the previous ones. The power level scaling killed any use they had in combat, they were doomed to become "useless characters" sadly.
With that being said, Toriyama could have probably handled their situation better, especially when he had to make up some bullshit like "Yamcha cheated on Bulma" to make her break up with him so Trunks could exist, when we know Yamcha was terribly shy with girls
