User blog comment:CoolJazzman/Why do you think Yamamoto wasn't giving a promoted to become a royal guard./@comment-5551986-20130417192515

Other people have stated him as, irreplacable. From what we have seen of the Royal Guard, despite the fact they are all apparently very powerful, is that they have all attributed a creation of immense importance to the workings of shinigami and soul society. From what we know, Yamamoto bought forth an organisation. Not an invention. He may have been the oldest shinigami (although I have a feeling Óuetsu Nimaya or whatever might beat him in that regard, im inclined to think the Oken bestows everlasting life to the wielder) so Yama-Ji already knows the history of some of Soul Society But it wouldn't make sense to promote him to the Royal Guard for his power alone. The Gotei 13 and Academy can always be run by different people. Its like me starting a political party, take Labour for example in the UK. The ways its run may change throughout generations, but it still has the same concept. This is the same with the Gotei. Theres no point promoting Yamamoto to a position he would not be better off in. I doubt many can run the Gotei as well as he did. Why would he want to abandon his own Idea?

Anyway, hes was the strongest shinigami, he would have easily been on par with the Royal Guard. So far the only way anyone has beaten Yamamoto has been through trickery, and trickery alone.