Board Thread:General Discussion/@comment-26091332-20160820062353/@comment-27392440-20161021173832

Though I can't prove my theory take it or leave it until disproven but one very real possibility is that he lost both quincy and hollow powers but retained a pure version of his shinigami powers. The logic goes like this: the hollow White mixed with whatever his zanpakuto spirit should've been and made it hollow-like/ the hollow and quincy spirit were in a very real way external to Ichigo's spirit (white was an artificial hollow and old man was part of Yhwach) which meant some part of Ichigo's soul had to be truly his own that he wasn't in touch with/ when Yhwach stole his quincy and hollow powers he inadvertantly left Ichigo with only the power that was from himself, sort of "purifying" the original zanpakuto spirit back to what it should have been and leaving Ichigo only as a shinigami. I think the symbolism of his "bankai" going back to his original shikai may have had 2 meanings as well: 1 it signified Ichigo being back to only a shinigami like he started, without all the upgrades/taints his other powers mixed into it, 2 that that could've just been his shikai, and not his bankai all along, but that it was bankai level from the other powers mixing with it (that's supported by, if I remember correctly, Ichigo's garb not changing like it always used to when entering bankai, that Byakuya hinted that there's no such thing as a constant release type like Ichigo still seemed to have, and that he technically never mastered bankai as he never communed with his true zanpakuto spirit to fully release it). Hope the theory of mine gave something akin to an answer until Kubo gives something even more convoluted to process