Board Thread:General Discussion/@comment-24098614-20141011105958/@comment-5341204-20141011130158

First off, for Ichigo to eventually beat Yhwach, he'll have to be more powerful than Yamamoto by a fairly significant margin. Yhwach killed Yamamoto in battle without too much difficulty, and Yamamoto is a far more practical fighter than Ichigo is - if he saw a weakness he could exploit for an easy victory, he'd take it. Ichigo likes to take down his enemy on his best day.

Secondly, Yamamoto would only ever have entered the world of the living under a significant limiter or during the Fake Karakura battle (and the dimension that battle took place in is kinda odd - the pillars did something to render Karakura Town inaccessible to Aizen - essentially swapping his intended destination with a facsimile of the city that had been built in Soul Society specifically for that purpose. As I recall, this was achieved by actually transporting the real town - and all of its inhabitants - to Soul Society and transporting the facsimile into the world of the Living.  This was maintained by 4 pillars that were protected by lieutenants in the early stages of that battle.  Such pillars probably also served to protect the rest of Tokyo (and points beyond) from the fallout of the battle.

Thirdly, Yamamoto is far more restrained in the use of his abilities, and was a trusted senior officer among the Shinigami. Ichigo, on the other hand, is considered something of a rogue element - an asset that's useful when enemies need smiting, but dangerous and unpredictable at other times, plus he lacks the inherent discipline necessary to hold himself back to the degree that Yamamoto did.

As for the ending, who knows - we know Ichigo lost his power once and lived as a normal human for quite some time after defeating Aizen. Perhaps Urahara will offer up some new kind of limiter that will scale Ichigo's power down enough that it doesn't disrupt the world of the living as described.