Board Thread:General Discussion/@comment-24098614-20140927081958/@comment-5398287-20140927141205

Shunsui never said he could never go back to the Human World without a doubt, no matter what, etc. etc. - he said there was a chance, a very small one, that Ichigo would come back from the Soul King Palace so powerful that his mere presence in the Human World would be massively disruptive to the Souls around him. Remember, captains and lieutenants have to wear power seals while in the Human World in order to prevent their Reiatsu from adversely affecting regular Humans, so imagine what someone like Ichigo might do if he can't keep it under control.