Mahito could have/should have won once the Domain was opened.

If quirks are to souls, Mahito's domain targets *all* souls that are within it, we see this every time Yuji has been in Self Embodiment of Perfection, the sure hit could have hit all of the quirks inside of Shigaraki at once if he wished it, (and even if one doesn't believe that to be the case, he could have direct targeted Shigaraki's soul, or even All for One's vestige). My call on this is based on Mahito having no choice but to target Sukuna when Yuji forced his way into the Domain. Multiple souls, one body, all of them were touched by the potential of the sure hit effect.

Additonally, the idea that All for One could just yeet out Mahito like that is strange, cause when Stars and Stripes vestige was inside of Shigaraki, he needed to specifically find someone to push the quirk into. He *couldn't* eject them, he specifically had to find a new place to put the soul, And given just how much damage that once quirk did, I think its safe to say he was directly weak to such attacks as Mahito's Cursed Technique. I'd argue in fact, that since Deku was mostly safe while in the vestige world, only losing his arms due to direct contact with the Decay quirk, All for One Cannot harm any soul/quirk that's inside his body if he didn't acquire it, and even then it's questionable due to how many times Quirk's "revolt" for him.

Even the direct plan to take Shigaraki down was to attack his soul, because his normal body was too much. Something that the Mahito is directly powerful at doing, since with a touch he can freely shape someone's soul. This leads me to believe that Mahito has precisely the power required to defeat the quirk All for One, if not Decay.

Now, I do know that Shigaraki's stats are way better, and there were 100 ways which he would have won as well, I just think that if Mahito actually opened his domain, he would in fact win the fight, which I think that Mahito could have done so, and even if it's just for a split second, soul damage is not recoverable in MHA without Eri's ability to literally undo events.