this also could unfortunately lead to a Shigaraki redemption, depending how the vestige of AFO acts.
I think the real villain was always AFO and shigaraki was merely his pawn.
I think it will probably be revealed that AFO gave shigaraki decay purposely because he wanted shigaraki to become a villain. He never really cared about shigaraki and only needed him as a contingency plan.
Ujiko's life force quirk doesn't give immortality, it merely extends one's lifespan. Hence, AFO was always aging and this must have been a problem for him. This was the entire reason he needed shigaraki.
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Shigaraki will realize this, along with the fact that AFO killed his grandmother. This will lead him to giving up his dreams and asking Izuku to save him.
Izuku will need to find a way to stop AFO without killing shigaraki.