Akainu only faced Old Whitebeard when he was already sick, stabbed and mortally wounded and eventhen in 2shots he was already close to KO. Can't even imagine what Old Whitebeard would have done to him if he was fresh. Not to mention that Prime Whitebeard should be much stronger than Old sick, stabbed mortally wounded Whitebeard, leaving a huge gap between Prime Whitebeard and Akainu.
Akainu like i've said has no chance to become final villain if even after defeating him, there is still that huge of a gap between him and Prime Whitebeard that could have been filled by the likes of Garp, Shiki, Prime Rayleigh and so on. Prime Rayleigh for example implied he could defeat Kizaru(slightly weaker than Akainu) without much difficulty(despite his back-ups) and not only he is weaker than Prime Whitebeard but is destinated to be surpassed by Zoro. So Zoro himself should be much stronger than Akainu by the end of serie. In a long time frame frame, Akainu level just won't cut it and Akainu is already past his prime(55 years old). Hard to see how he will evolve that much