Erkan is right; Blackbeard himself never had a grudge against Ace, wanted him to join and even after Ace refused, being rather gentle to him, delivering him to the Marines and telling Luffy in Impel Down that he won't say it's impossible to save Ace. Meanwhile Akainu trash talked Ace' father figure and killed Ace when latter tried to protect Luffy. And with Sabo inheriting Ace' power and will, he just like Ace will stand between Luffy and Akainu, but this time with a different outcome.
Also this. Luffy never wasted a thought on Akainu in particular, he just said "I don't care whether it's an emperor or admiral, I need to surpass you all", a statement made while he was facing an admiral. So with Luffy surpassing Kaido and later even Blackbeard, I don't really see how Luffy has to prove himself by beating Akainu?