Sabo has reasons to defeat Akainu: Ace's death, Kuma's probable death
Also, we'll have fire vs magma fight again
Round 1: fire vs magma (Ace vs Akainu)
Round 2: fire vs magma (Sabo vs Akainu)
It's more fitting than Luffy vs Akainu, even if both are possible
People who believe that Akainu has to definitely face Luffy are the same ones who believed that Luffy would definitely take Big Mom down