But haki transcends all

While all of this might sound good on paper and I agree with it to a certain extent, it would still not make him able to damage a HK or one of the Gorosei though. And that's the big issue.
For example, just make Sanji fight someone equally strong but with no gimmick. Like, Zoro fights the Elder and Sanji fights the Admiral. As long as the end result will stays the same (top tier opponent).