It was a cheap shot, then a huge attack. Akainu managed to fatally injury an old, sick, tricked Whitebeard but just fell down and did nothing for some minutes.
Alive only for plot armor, cause in that situation just couldn't defend himself anymore, he yelled at and cursed WB after being temporary defeated.
On the other hand, I don't know, even after all these years, how the fight would have gone if a WB not tricked and already injured and Akainu just fought alone, in a neutral field, like with Aokiji in Punk Hazard.