I mean if people really do believe that the Marines would actually be antagonists to the SHs at some point then isn't it pretty obvious that Akainu who's likely to be Luffy's opponent would be significantly stronger than all the others? Moreover, why are we ignoring the fact that this is ultimately a shonen manga? The higher the rank the stronger the opponent is how it usually goes. Hell we quite literally have Oda giving Akainu a whole 5bil bounty in comparison to the 3bil for the other Admirals. Not sure why we are overcomplicating this.
Personally as a Sanji fan, I don't really mind Kizaru or any of the other Admirals for that matter being Akainu's equal as one of them is likely to fight Sanji if we do get SHs v Admirals but surely that isn't going to be the case. If anything if the Admirals are indeed all equals then you really have to ask the question whether they are actually going to be standard antagonists for the SHs.