It's not PIS that Yama didn't beat Bach. 1. His bankai was stolen (his strongest power), 2. He was obviously fatigued by his scuffle with Royd, 3. Bach is around Yamamoto's own level if not stronger. He can point at non fodder characters and make them explode or take an arm off and what not. Yamaji is by his own statement and Unohana's a world destroyer, and in real life physics, something as hot as the core of the sun at the size of Yamamoto would cause global damage. The problem is I don't see Zebra losing regardless. The nature and scale of his own techniques would allow him to beat Yamamoto the majority of the time first. Zebra himself is already a confirmed country buster. Even Sani is a potential planet destroyer and Zebra is the most powerful King. Not saying that Zebra can bust a planet (yet), but that he is still powerful enough to one shot Yamamoto and vice versa. But he can do so with greater ease and more quickly. Even assuming Yamamoto is a planet buster/life wiper, his durability threshold while impressive isnt near that.