“The quest stands upon the edge of a knife. Stray but a little, and it will fail, to the ruin of all.”
Galadriel, The Fellowship of the Ring
The following contains spoilers from chapter 304 of My Hero Academia. You have been warned.
The latest chapter has confirmed one of the greatest suspicions. It looks like Deku really is the final wielder of One For All. This was confirmed when Deku had a meeting with the vestiges of the previous wielders of One For All. During this meeting, we got to understand the true nature of One For All and why the previous wielders died young.
As stated by the fourth wielder, One For All is ill-suited for someone who already has a preexisting quirk. The reason for this is because One For All tends to burn away the life-force of a wielder who already has a quirk. This causes a sense of overload in the wielder’s body and causes them to age at an accelerated rate. This was the case with the fourth wielder, who died prematurely at the age of 40. Because of this life-draining side effect, the previous wielders of One For All could not use the quirk’s full power.
However, All Might was the only wielder who could use the full power because he was originally quirkless. A quirkless person is a blank slate with no preexisting quirk to hold them back. This makes them immune to the life-draining side effects of the quirk and use One For All as though it were their own natural quirk. This was why All Might was able to wield One For All for 40 years and become as powerful as he was.
Deku is in the exact same situation as All Might because he was originally quirkless as well. The fact that Deku is manifesting the quirks of the previous wielders is not just because of the stockpiling nature of One For All. It is a sign that One For All cannot be transferred anymore. Also, with so many people possessing quirks, quirkless people are becoming a rarity with each year that passes. Since only quirkless people can truly access One For All’s true power, the possibility of Deku finding a quirkless successor is becoming increasingly non-existent. This is another reason why the quirk cannot be passed on anymore.
Overall my theory of Deku being the final wielder of One For All has been confirmed. That makes him One For All’s last chance to bring down All For One once and for all. If Deku falls, any chance of stopping All For One/Tomura Shigaraki will be lost.