Sabo's chances of fighting Akainu have gone down considerably imo. Now that Shamrock exists, it's looking more and more likely that there are atleast 2 top tiers linked to the Holy Knights (Garling and son), which conveniently fills both slots for Dragon and Sabo (kind of a father son combo of their own).
Luffy vs Akainu was always set in stone thanks to the scar. It was just a matter of whether Luffy sees it all the way through to the end, and yea with the prominence of the Holy Knights, Sabo has effectively been given a legit role. So Luffy is probably going to be the one to not only fight, but also defeat Akainu.