Remnants remaining would be fine... it still doesn't change the fact that Ichigo got power drained, Showed he was barely standing and still had enough power to kill Yhwach... Aizen giving him opening be damned
It really depends on how you want to view everything. If you go by Yhwach's feelings towards Ichigo, Ichigo is his son symbolically and thus the son of God. The son of God should be able to kill the father. Ichigo could be on a level where even in a weakened state, the overall power of his being and unique soul composition make him a divine being, in a sense.
I've said this numerous times. This wasn't the end Kubo had in mind, considering he made Ichigo have a new bankai for absolutely no reason. But it's what we got.