All for One is the most powerful superhuman with the exception of All Might who has the quirk One for All. If All for One has the quirk who makes him able to steal quirks, and All Might is the only person he can't defeat, why All for One doesnt steal One for All and let All Might quirkless to be able to defeat him?
First, What is One For All?
One For All is the union of two Quirks: A Quirk that stockpiles power, and a Quirk that could be transferred to others. This mutation was caused by the Quirk, All For One.
One For All cannot be stolen by force:
One For All will not transfer to a new recipient unless its user wishes it, thus One For All cannot be forcibly stolen. For example, the Hero Killer Stain ingested Izuku's blood, but did not inherit One For All because Izuku did not wish it. However, this does not mean One For All cannot be forcibly passed on.
Why All for One doesn't steal One for All and let All Might quirkless to be able to defeat him?
Apart from the reason why can't One For All be stolen, you can see that:
After One For All is transferred to the recipient, the previous One For All user can still use the "leftover embers" of One For All, and thus can still access One For All's power. However, these leftover embers will not last for long, and the previous One For All will slowly run out of the embers, resulting in the previous user losing One For All completely.
All Might has the "leftovers" of One For All but soon or later he will loose One For All completely, the user with One For All now is Izuku, if the Quirk was to be taken it had to be from Izuku and not All Might.
I think there is a combination of reasons:
- Originally, One for All was a completely useless quirk that would have been undetectable. All it did was allow itself to be passed to someone else. This quirk was combined with another quirk that allows the user to store up power and use it as super strength, courtesy of All for One. All for One actually was the one who gave the original possessor of One for All the second quirk, most likely not knowing of the existence of One for All or that the two would combine in such a powerful manner.
- Once One for All became as it is now, it would have been difficult for All for One to simply take it. The owner of One for All would have resisted using his now quite powerful quirk. Also, in the beginning, when One for All had less energy stored, All for One also probably had stolen fewer quirks. If he took time to steal more, One for All would have become more powerful by storing more energy. So they may have been evenly matched at each encounter despite them both acquiring more power over time.
- One for All only is as powerful is it is now because it was passed down by many users who all stored up energy using One for All. This means that immediately after gaining that second quirk, the original owner of One for All would only have seemed exactly as expected, rather than unexpectedly powerful, as it turned out to be. Remember, All for One gave the original owner of One for All the second quirk. He intended for him to have it and use it, back when it was less powerful. Only now that it has passed to others has it become apparent how much potential the two quirks had.
- Furthermore, it might have had to pass between many people to maximize its potential as its not entirely clear if a single long lived person could attain the same result. I don't think One for All would even necessarily be interested in a quirk that he had to share around in order for it to reach its maximum potential. Perhaps now that it has reached this level of power he would be interested in stealing it if he could, however.
There are ways to get around the limitation of one for all not being able to be stolen by force, by using a mind control quirk for instance. We already saw Deku being controlled by Hitoshi Shinso, so in the future, a new All for One user might overtake the One for all user with a powerful Mind control Quirk, and force him to give up his power, ushering in an age of destruction. The problem with Hitoshi Shinso's power is that it's not really "Mind control" but body control. A True mind control quirk would make the person controlled actually believe that they want to do the things they are being forced to do after the trigger condition is met, so resisting would basically be impossible. This would make for the most Nefarious version of All for one, where in the final battle, One for all thinks he has the upper hand, when he triggers the mind control condition, and is forced to give up his power, probably all on Nationwide television.