A lot of people have pointed out the reconning that was done for this arc, and I don’t think anybody seriously believes Kubo planned this out ten years ago from the very beginnings of the series. That said, I think it was done relatively well enough to seem plausible, and I don’t find it to be a huge distraction. It was written so that a lot of things lined up and there are no glaring inconsistencies, but at the same time there was no foreshadowing of this twist at all before this arc and Zangetsu was treated as the real zanpakutou all along, including all of the times he has trained and gone into his inner world, such as for his bankai and final Getsuga Tenshou. The only real explanation I can see for this is simply that he was a damn good imposter.
As Ichigo confronts him, he explains that he is actually a manifestation of Ichigo’s quincy powers (hence the resemblance to Yhwach), and that he’s been suppressing Ichigo’s shinigami powers in an attempt to save him from Yhwach himself. Ichigo’s inner hollow has been responsible for saving him a couple times too, and contributes to a good portion of his power. In an unfortunately quick turn of events quincy Zangetsu decides that it is time to say goodbye as he dissolves into a sword in the shape of a flaming cross. Whatever his new zanpakutou is going to look like, I’m quite excited at seeing what new abilities he will have, as he is implied to be much, much stronger now without the restraints on his powers. Just as I was wondering about how Ichigo could possibly get stronger after all the power-ups he’s received before, Kubo manages to outdo himself yet again. However, I am a bit sad to see the old man go, especially after he shed a single tear at the very end. It’s not clear what all this means for Ichigo’s quincy abilities, but it is possible that he will remain with Ichigo in some other form.