Sabo fighting Akainu alongside Luffy with Zoro fighting Kizaru instead.
Hm, I still don't think Sabo is fighting Akainu.
I can see Sabo fighting one of the Gorosei because the system they created fucked him up, but not Akainu. I feel like Sakazuki is way too personal for Luffy to let anybody else interfere. Any outside help Luffy gets against Akainu will solidify the fact that Luffy would always be powerless against Akainu. That's why it's essential for Luffy to defeat Akainu alone. Luffy has something to prove to himself against Akainu, while Sabo does not.