The following content contains spoilers from My Hero Academia, Season 6. Deku is being hunted for his Quirk, more extensively in Season 6 than ever before, as a hoard of All For One's ruthless minions try to incapacitate the young Hero at every turn. However, One For All can only be willingly transferred, so what's all the fuss about?
Fans have been afforded the opportunity to meet all of One For All's Past Users and understand more about the mechanics of this distinctive Quirk, and it seems that All For One may have bitten off more than he can chew. Simply wielding One For All does not grant the User automatic access to its powers, as Deku discovers, the hard way.
One For All's "Steal-ability"
The existence of One For All is no longer a closely-guarded secret, although, it seems not everyone is aware that this multi-powered Quirk can get passed on through the generations. However, even if this was common knowledge, One For All has a safeguard in place to prevent this potent Quirk from being stolen and falling into the wrong hands. One For All cannot be relocated unless its User voluntarily activates the transfer (and adds a little bit of their DNA to the mix.)
Deku seems particularly fearful of having his Quirk stolen by Nine in My Hero Academia: Heroes Rising (confirmed to be canon), which doesn't make sense considering he is well aware that the shift of power would be unsuccessful without Deku's approval. Nine assumes that he is unable to steal the boy's abilities because his own Quirk capacity is full, and seemingly sets his sight back on Katsuma Shimano, leading Deku to willingly share his powers with Bakugo to gain the upper hand. However, Deku can't seem to shake the unnerving feeling that he nearly lost One For All to the enemy that day, which suggests that it could possibly be stolen. Or maybe the boy simply panicked in the moment and actually had no reason to fear.
The Protective Past Users
When Nine gets his hands on Deku in Heroes Rising, an image of One For All's Past Users flashes across the screen, just before the tyrant's connection to Deku is broken, subsequently stopping the transfer in its tracks. Nine may just be a prototype, but his ability to steal is assumedly a direct copy of All For One's Quirk, so why would he, or Shigaraki, produce a different result? Deku didn't gain instant access to all of One For All's Quirks, with the Second and Third Users putting up a major fight when it came to sharing their powers. So surely, all Nine Users would simply refuse to cooperate with All For One, too?
The Symbol of Evil tried, and failed, to rob Daigoro Banjo and En of their abilities before, so something must have changed for him to become so persistent again. Perhaps Shigaraki holds the secret weapon for success.
"One For All can only move on the will of its owner as a rule, but your anger is starting to encroach on it."
The tyrant seems to believe that by combining powers with Shigaraki and his potent hatred, the pair could overwhelm One For All and claim their prized possession. But again, this is yet another manipulation tactic, one which has already failed once (so far.)
Even if All For One somehow manages to convince Deku to part ways with his powers of his own volition (maybe via a hostage situation or some other conniving scheme), there is little chance of persuading the other eight Users to surrender as well (besides maybe the guilt-riddled Nana.) Fans have seen firsthand how stubborn these veteran Pro-Heroes can be, and how much they deeply despise All For One. These former Heroes have personal experience with what a lawless society looks like, and would rather die fighting than admit defeat and essentially assist the Symbol of Evil in taking over the world. Brute force or fancy Quirks aren't going to work here, so is there really anything worth worrying about?
All For One's Forced Quirk Activation
In a hypothetical situation whereby All For One has taken ownership of Deku's powers (but the Past Users are completely uncooperative, rendering the Quirk useless), another argument could be made regarding Forced Quirk Activation. All For One rarely needs to sweet talk his way to success, as he simply asserts his dominance with this nifty Quirk. So technically, he can whip the Past Users into line with little effort, problem solved!
All For One regularly uses his Forced Quirk Activation in conjunction with Rivet Stabs; however, it is unclear if this is for convenience's sake or out of necessity. If the latter is true, then All For One surely loses the advantage of his Forced Quirk Activation when it comes to One For All, as he can't exactly Stab a spirit or "consciousness", now can he? Except for Izuku and All Might, none of the Past Users have physical forms (presumably) which suggests that this tactic will prove to be ineffective unless Forced Quirk Activation can be essentially self-inflicted or used within the otherworldly realm (of Season 6, episode 10) where One For All and All For One had their previous confrontation.