The top reasons for why a lot of people handwave this blank area have to do with the infationary zenkais in the latter half of the Freeza arc and the Super Saiyan multiplier making things too numerically difficult for the humans to reasonably catch up.ruler9871 wrote: ↑Mon Apr 22, 2019 7:37 pmI've never understand why Goku nor even King Kai (who trained most of the Humans) never taught any of his allies Kaio-ken. It makes zero sense, especially in King Kai's case.Shaddy wrote: ↑Sat Apr 20, 2019 8:23 pmWell if you want to fight against saiyan-only transformations, utilize and improve the transformations that aren't specific to saiyans. Ultra Instinct is obviously the most recent example of this, but there's also stuff like kaio-ken and...well no, that's really the only two, and kaio ken isn't really a transformation so much as a technique, but there's no reason for the others not to be able to use them other than that Goku is soooo strong OwO and everyone is sick to death of Goku not just being the best but the show actively jerking him off at every opportunity.
It's likely just another example of Toriyama being a neglectful writer, since Toei (especially with Heroes) & Toyotaro have no problems giving new powers & abilities to the side characters in order to catch up.
I won't deny that those developments helped to skyrocket the Saiyans into the aeons, power level wise. Human fighters like Yamcha and Tenshinhan would need something amazing to keep them competitive. From my personal viewpoint, I think a combination of tapping into the human body's pinnacle strength, i.e hysterical strength, with a Kaioken amplifier that surpasses Goku's x20, it might be enough to put them on-par with or even surpass the Saiyans briefly. Something like that would only make them competitive for just a burst or two at best. It's a self-destructive means of gaining power that would end up devastating the person's body.
Is this going to happen? Probably not. Saiyans have been at the forefront of everything since the Cell arc. New hair coloured transformations for only two characters is easier to write for than devising new and potentially complex ways of keeping less favoured characters viable. Many of the fans probably aren't interested in what happens to the likes of Tenshinhan or Krillin. The only non Saiyan who seems to be desired for making a comeback is Piccolo. We don't see him trading blows with Goku Black or Toppo, though, do we?