since One for All can't be transferred now to people with quirks
This isn't entirely true. Even with One For All becoming extremely powerful, there is no hard limitation against giving it to someone with an existing Quirk that we know so far.
The rationale behind preferring quirkless people is noted by the Weaknesses section of the wiki as follows (emphasis mine). I'm putting everything under a spoiler just to be safe, because Mirio regaining his quirk is Chapter 293 and most of this information is discussed in Chapter 304, though it is foreshadowed in Chapter 284. If the anime has some episodes left this season, it might get covered.
Following the research conducted by All Might, as well as the
discourse of the One For All vestiges, it is revealed that One For
All can affect the user's lifespan. If a person who already has a
Quirk receives One For All, the extra Quirk places an enormous burden
on their body, depleting the user's life force and shortening their
It is analogous to adding water to an already-full cup, causing it to
overflow and breakdown. The rate at which the person's life decays is
unknown, but Hikage, the fourth user, received One For All at the age
of twenty-two and died of "old age" eighteen years later, at the age
This weakness can be circumvented if an inheritor does not
already have a Quirk, since the Quirkless user would be an "empty
cup" that is filled by One For All.
While past users have all had existing quirks, it seems that All Might's research is what made them aware about this side effect. Whether they would still pass it on to someone with this knowledge is speculation, but nothing is preventing it and I'm not sure if any of their approval is needed beyond the current wielder who has to willingly pass it to a successor.
If the quirk removal was permanent, then
by the water analogy the cup has been emptied
and the user should be able to use One For All without the side effect.
Again, the caveat is that there might be more factors that nobody knows about, like whether the mere possession of an original Quirk alters the body in a way that the removal doesn't change. We know Quirks are genetic and there's an in-universe theory about their singularity, so a lot can change by the end of the series.