The bullet made by Chisaki reverses a human back to the time when they were Quirkless. In theory, then, shouldn't Mirio's Quirk come back on its own as time goes by, just like when one's Quirk first appears?
No, it was mentioned the the bullet that hit Mirio was a permanent one. That means his quirk is gone for good unless someone reverses his body to the state before the quirk was lost. That is why they are helping Eri control her power so that she can help Mirio get back his quirk.
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