In my opinion, toriyama was told to make goku more superman like.
See, toriyama isn't really into DB as much as people think he is.
He was quite honestly beaten into the ground by his editors and schedule and he basically did what he needed to do.
That frustration and flying by the seat of his pants made DB what it is.
Some part of him wishes that it was more childlike and fun, but that was taken from it by the greed of the suits.
You can look at DBS and some of the new material and see the fingerprints of toriyama and see which decisions and stories were crafted for money by people that weren't toriyama.
SSJ forms outside of 1 are just mutations of SSJ1. There is only one ssj and there is only one legendary ssj(let's just ignore nu-broly and pretend that og broly minus the kakkarot thing or cumber is the legendary super saiyajin from now on okay?)
S-cells? Toriyama was tasked to make an excuse for the gay ass u6 fuck up.
Toriyama was tasked with creating Cauli to make up for the she-broly fuckup that some retarded suit thought up
SSJRed was just Toriyama creating a temporary form(looked shit but was also a slight nod to nozawa. look at that red hair) that was never supposed to be used again. He had goku outright state that he hates having to do a ritual like that to gain power, his body remembers the power level of the ritual and he didn't have to use any other super saiyajin forms anymore, AND that beerus was pushed to use almost 70% of his actual power and that there were more powerful gods and super being LIKE BEERUS NO MULTIVERSE in the universe to fight.
Then RoF came in and just ass fucked the entire world and Super was created and it's just been a slide into fucked upland ever since.