Board Thread:General Discussion/@comment-5695542-20131107215511/@comment-1268838-20131111140752

Strictly speaking, if Ichigo's assessment of him was correct (as I find likely), then it was ultimately Aizen that caused his own defeat. Aizen subconsciously desired an opponent that could match or surpass his own power, which is why he was so keen on allowing Ichigo to develop, and took an active role in fostering that development. When Ichigo ended up surpassing all of his expectations, Aizen's innermost desire was satisfied, and he no longer had a reason to be a so-called "transcendent being." Since the Hougyoku's power was all that restrained the seal, he became vulnerable to its power.

Granted, Ichigo and Urahara were both essential in the end. Ichigo to satisfy Aizen, and Urahara to clean up the mess - as an immortal, he probably could not have taken him on once Ichigo was out of gas.