We have seen Goku achieve some pretty amazing feats since we first watched Bulma shoot him in the face while searching for the Dragon Ball’s. In that time, Goku has awakened 11 canon transformations in that time and has defeated numerous threats to the Erath and the universe. However, due to some precautionary events, no matter how much Goku trains or how many new levels of power he reaches, Goku will never awaken his full potential.
The reason for this? Well, it’s actually a theory that the non-canon Dragon Ball GT explored but couldn’t be more accurate and it’s affecting Goku in Dragon Ball Super now. For those who may remember Radiz’s entry to Earth when he spoke to Goku, the older brother was shocked that Goku had removed his tail permanently and that it was the key to unlocking a Saiyan’s true potential.
Now, yes, we all assumed that Radiz meant that it was a key to Goku’s Great Ape transformation and in a way, it was in reference to that. However, there’s more to Radiz’s words than most realise. I went further back to the original Dragon Ball when Goku first entered the World Martial Arts Tournament and his tail reappeared. It was episode 23 of Dragon Ball and Goku himself said that “he always feels stronger with his tail.”
Therefore, without Goku’s tail, the Saiyan is slightly weaker. As I said, Dragon Ball GT did explore this when they introduced the Super Saiyan 4 Transformation but that’s sadly non-canon and so, all canon content has Goku disadvantaged by the loss of his tail.
In addition, this also means that all Saiyan’s on Earth such as Vegeta, Gohan, Trunks and Goten are also disadvantaged just like Goku is. It’s possible that if everyone kept their tails (we assume Goten and Trunks had them as some point) then all of these characters could be significantly stronger.
I understand that at the time, the Great Ape transformation of Goku in Dragon Ball and Gohan in early Dragon Ball Z, was too dangerous to allow to be set loose. This is why Goku and his friends had to remove the tails although, in hindsight, they could have been a key to more power if Goku and Gohan learned to master that power as Vegeta did.