Son Goku's most powerful transformation that was introduced in Dragon Ball Super's final story arc of its recent anime adaptation is definitely Ultra Instinct which was a technique that rivaled, if not surpassed the power of the Gods, and we think that it's a level of power that he should never master. During the Moro arc within the Akira Toriyama penned manga, Goku is clearly moving in a direction of learning all the intricacies of the transformation and we can't help but think that this is a mistake for the character of Son, if not the franchise as a whole.
It's The Plateau
Quite frankly, would there be anywhere else for Goku to go if he was able to completely master the form of Ultra Instinct. With this transformation not relying on the previous forms that are Super Saiyan, the protagonist of Dragon Ball ascended to the power of the gods with both Super Saiyan God and Super Saiyan Blue and Ultra Instinct pushed him even further. Goku is practically within striking distance of Zeno, who is essentially the "god of creation" and if the Saiyan were to surpass him, what could there be left?
Goku Needs Challenges
The Tournament of Power allowed Goku to access a new level of god-like power over the course of forty five minutes, and with the manga seeing Goku having already mastered the ability to use "Ultra Instinct Sign", the first level of the technique, it seems as if the final plateau for him to achieve is within his grasp. At this point, Moro is certainly a villain that has the ability to give Goku a run for his money, but little others can. Does becoming more and more invincible make the Dragon Ball protagonist boring?
Goku Ascending Past The Gods Isn't The Right Move
Goku is a strong being, but becoming more powerful than the gods almost seems like too much for his character. Absent headed and child-like, the Saiyan who was shot to earth has always loved fighting more so than anything else but would his character wither on the vine if he were to surpass actual deities? When Goku fought against Beerus in the opening story line of Dragon Ball Super, their blows effectively began tearing open rifts in reality, and while that problem hasn't reared its head in future fights, would Goku becoming stronger than the Gods tear the universe apart? If not, it would definitely seem like a continuity error.
It Solidifies Goku As The Strongest
Goku has been the number one fighter for some time now in Dragon Ball Super, but honestly, the series is often more interesting when other Z Fighters have managed to ascend to a level of power that makes his pale in comparison. After absorbing Kami during the Cell Saga, Piccolo's stint as the strongest fighter was downright captivating, to say nothing of the brief period of time where Gohan had achieved the Mystic level and became the strongest. With Goku achieving Ultra Instinct, can anyone even come close on the hero side to touching him at that point?
It's Vegeta's Time
The Moro Arc has given Vegeta a unique new fighting style by having him train on the Planet Yardrat, the same locale that taught Goku how to use Instant Transmission. With the prince of the Saiyans seemingly having gained new abilities and a significantly higher power level on the alien planet, fans are crossing their fingers that he will be the one to bring down the ancient sorcerer! If Goku learns Ultra Instinct, is there any hope at all of Vegeta finally being able to surpass him in overall power? The chances of that are basically nil!
Ultra Instinct Shouldn't Be Like Other Transformations
Goku has learned how to master every transformation he's ever been able to access. From Super Saiyan 1 to Super Saiyan Blue, the sheer amount of techniques he can use to give himself a higher power level and a nice new color of hair has simply been staggering. Ultra Instinct should be wild and untameable, something that even the gods fear, and while Goku has managed to master the first transformation within it, the full power of UI should always be outside of his reach!