Or only the Akainu will have it. By beating him Luffy proves his superiority without the other Admirals needing CoC, they are below Akainu anyways.
I think Sabo will fight Akainu, it's more thematic anyway.
Plus G5 could ruin the vibe. Luffy is always laughing and using goofy techniques with it, even though this is gonna be a very serious and dangerous fight for him.
Sabo is more suited for this. He fought Fujitora before so we know he can at least somewhat handle an Admiral, and he has the fruit of his deceased brother who previously failed to even scratch Akainu.
And Sabo is more lethal than Luffy as well. He likes to crack people's skulls and what not, while Luffy just punches and kicks.
He's the perfect candidate for the fight. He can give Akainu the cruel yet poetic death he deserves.