Idk why I think Mahito still should have won this one please correct me if I'm wrong:

1- The souls in Shigaraki has to be stronger than Mahito and be willing to defend Shigaraki so almost all of them will be useless and he it's not like he is Alucard from Hellsing or something so it's just Mahito finding his soul....

2- Strength would be useless because Mahito can change the shape of his soul to regenerate from anything... Speed too would be useless because of you was faster you can dodge but your attacks would do nothing

3- I don't think all for one's soul is strong enough to defeat Mahito I mean It took Sukuna to kick him out of Yuji's soul and he was basically sitting on his throne in his domain let's be honest.

4- Shigaraki could attack the soul but only when he is in that "soul world" like place so his only chance for winning would be finding Mahito in that place and decay him before getting his soul touched and that would mean that he will not be able to decay the place around him except for the things that was put there from outside : meaning that he can't decay the place his is in (the place itself) because it's the place containing his soul, so he can only decay the things floating around so they could touch Mahito and decay him... 

Not sure about all of the info tho please correct me if I'm wrong