But he's not though lol, have you read the last arc? Dragon aims to attack the God Knights, and now we know the Gorosei and Imu are near immortal beings, let alone the fact that CP0, Kong and whatever else still exists in Holy Land.
Id sooner expect Akainu to actually be Elbaf's villain or part of the big Yonko battle afterwards, then him defend Mariejoa..
In fact, that would be the right time to do it. If the Marines mobilize and head towards Elbaf or Lodestar, then Mariejoa is free for the RA to attack it (until Imu obviously uses Uranus)