ok,considering the fact they get Deku's blood(during the provisory licence arc),so they have a sample of OfA,and in chapther 209 ,in the last page ,when Deku's other quirks started to awakening ,AllforOne said he could hear his own brother,so maybe he know about the conscience transfer properties of OneforAll,what if the Doctor is going to fuse this two quirks together somehow or just putting some properties of OneforAll in AllforOne,so Shigaraki could get all the stockpile power from his mentor,and AllforOne conscience could live through his own quirk,so even after death, he would live on Shigaraki or even take control of him if necessary(in a similar fashion of what Orochimaru tried to do with Sasuke),and would be fitting with AllforOne characther......giving away his quirk for a chance of eternal life virtually.....