When has that EVER stopped a mangaka?
If I remember correctly SSJ1 only comes around once every 10.000 years or something. Goku, basically the best martial artist of all time in DBZ world, only achieved it after dying himself, being trained by Gods and seeing his best friend being blown up right in front of his eyes.
After that all of a sudden there is SSJ2, SSJ3, SSJ4, SSJ Red, SSJ Blue, SSJ UI and so on. All of a sudden Goten 6 years old and Trunks 7 years old achieve SSJ3. Something not even Vegeta or Gohan, two ultra elite fighters, ever achieved.
Same thing in a lot of other mangas as well. If Oda (or rather Shueisha or whoever ultimately makes the money from the toys, movies, clothes, games and so on) wants to add G6, G7, G8, G9 and G10 they will. Milk the cow dry, it's rare for a mangaka to ever write two mangas with huge success. And with Odas deteriorating health this is his only shot at making sure his descendants 500 years from now all live in palaces.
