There are several reasons why Deku cannot pass on One For All. Firstly, let’s understand the nature of One For All. It is a powerful Quirk that accumulates strength over time as it is passed down from one user to another. Each successor inherits the combined power of all the previous users, making it an incredibly potent ability.
However, One For All comes with a drawback. As the Quirk grows stronger with each transfer, the body of the user must also adapt to handle the increasing power. This process is gradual, allowing the user to gradually acclimate to the Quirk’s immense energy.
When All Might passed on One For All to Deku, he did so knowing that Deku’s body was not yet prepared to handle the full power of the Quirk. All Might had already reached his limit and could no longer sustain the hero work that One For All demanded. He chose Deku as his successor, hoping that Deku would surpass him and become the Symbol of Peace.
Deku, being initially Quirkless, had a lot to learn and adapt to when he inherited One For All. He had to train extensively to build up his strength and endurance to even begin utilizing the power. However, even after gaining control over a portion of One For All, his body was still unable to handle the full power.
The strain of using One For All at its maximum potential was too much for Deku’s body to bear. We saw this during his battles, where his limbs would often get injured or break due to the immense power coursing through them. His body simply couldn’t handle the strain of using One For All like he could prior to that.
Even if Deku were to transfer One For All back to All Might, it wouldn’t change the fact that his body has a physical limit in terms of handling the power. All Might himself had reached his limit, and passing it back to him would not change the amount of time his body could handle hero work.
It’s important to note that Deku’s journey is not solely reliant on One For All. While it has given him a tremendous boost in power, it is his own determination, skills, and growth as a hero that will ultimately define his path. By honing his other abilities, such as his strategic thinking and support from his friends, Deku can continue to be a formidable hero even without relying solely on One For All.
Deku cannot pass on One For All because his body is not yet capable of handling its full power. Transferring it back to All Might would not change this fact. However, this limitation does not define Deku as a hero, as he has other qualities and abilities that make him a formidable force in the world of heroes.