Problem is, Yonko are ultimately silver medalists, they are supposed to be gatakeepers. Shanks could've been Pirate King, Luffy & Blackbeard will be Pirate King candidates and as such, they would be narratively above Navy.
But Kaido, Big Mom, Oldbeard, current Luffy and Blackbeard are NOT at the level where they can become Pirate King by their strength, so they are a fair comparison to other gatekeepers, such as Admirals.
Besides, I don't see how Akainu or Kuzan are any less important than Kaido or Big Mom. Akainu is so far the only character to break Luffy's will, in over 1000 chapters of One Piece. That is exceedingly important achievement, that I don't think any current pirate (except arguably Shanks) compares to.