Because it makes more sense for Akainu to be Sabo's opponent EOS than Luffy's.1. Hes not Kaguya, their characters very different. You can possibly compare him to Madara or Obito.
2. If the entire BB will be the final villain, because he had part in Ace dieing.. then why will Akainu not be the final villain? BB has done what luffy has been doing.. do w.e is best for themselves. Pretty shitty reason to be the Final villain..
3. That is IF BB is the strongest pirate.
4. No clue why you mentioned Kaguya, since it took 5 people to beat her.