I don't like the "it would disrupt the balance" argument. Yes they would disrupt the balance but they'd do so in a way that would make it so they could lock down full authority thereby establishing no need for "balance". If any one Admiral = a Yonko then the 3 of them in tandem would be able to wipe out a Yonko crew easily and shouldn't have needed the Shichibukai against WB, also if that were the case why couldn't they pick apart the yonko one by one? If you systematically wipe out the top 4 pirate crews you'll be able to wipe out any upcoming pirates before they could become a threat.The Marines can't defeat them, even if the Marines we're twice as strong as them, it would distrupt the balance. Do you read op?
Also Oda said if the mc was akainu he would put an end to one piece in under a year. He also said akainus df is the most destructive df of them all. He also tanked two haki enhance gura punches (and whitebeard strength wasn't weakened) he stalemated the world's strongest attack with a lava fist, and proceeded to take on all 16 of whitebeards commanders and be victorious. Yeah tell me akainu isn't on yonko level