Sabo literally joined the story because Ace died not before. Ace's death triggered Sabo to remember his past memories, if not for Ace dying, Sabo would still not remember anyone, not Luffy not Ace. Ace's death should be more important for Sabo in comparison with Luffy, because Luffy did everything he could, but Sabo did nothing about it, he even gets mad when Burgess says; ''what were you doing, taking a nap?''...
Then the first thing that he asked from Dragon to get Ace's DF power. Luffy only cared when Doflamingo told Luffy about it. At that point of course Luffy couldn't turn his back at it. The difference is, Sabo makes his plan to get it before, while Luffy just casually reacts at it when he has the opportunity to take it.
Sabo's fighting style is perfectly designed to fight against Akainu, in a direct clash fist to fist, Akainu can burn Sabo due to magma > fire unless Sabo has better CoA level than Akainu, however, by having the fire ability + Ryusoken, Sabo's resistance to the heat increased a lot more than the normal, meaning that heavily wounded and tired Ace's hand burned when he clashed with Akainu, but his right hand was still fine after the clash, it would be far worse if he wasn't using fire DF power. By using the fire ability, he can get close to use his Ryusoken techniques against Akainu in close quarter, without that fire resistance ability, the magma would be far more dangerous for Base Sabo.
Plus, now Sabo can move faster on magma territory, if he uses his fire ability to move and jump around Akainu's attacks. Which is better? Base Sabo vs Akainu or Fire Sabo vs Akainu? Definitely Fire Sabo, who has more resistance to the heat than normal. It's probably even better than some other DF powers, for example what Rubber does to magma? Or other paramecia DF powers? It's meaningless, not different than being in Base, but being able to turn into Fire, definitely brings more resistance to magma.
Sabo is no.2 of the Revolutionary, Akainu was obssessed with Luffy, because Luffy was ''Revolutionary Dragon's son''.
Revolutionaries will clash with the marines, Pirate Luffy beating a marine Admiral does nothing for the story.
Whitebeard beat Admiral Akainu, what's the meaning of it? Nothing. Pre-Yonko BB beat Fleet Admiral Sengoku, what's the meaning of it? Nothing. Pirate Rayleigh beat Admiral Kizaru, what's the meaning of it? Nothing. Pre-Yonko BB beat Impel Down Warden Magellan, still nothing changes, they just demote Magellan, and made him Vice-Warden, what's the difference if you make Akainu a Vice-Admiral after beating him? Nothing.
But if Luffy beats another Yonko, that would strengthen his position as the Strongest Pirate.
If a Revolutionary beats a marine Admiral, that will create more hope and sympathy for the Revolutionaries, people will start believing in Revolutionaries power instead of World Government's power.
But people are not going to start cheering for pirates if pirates beats marines, that will only motivate people more to fight against the pirates, since the pirates will always remain as criminals. Which is why the story of One Piece, is fighting against Yonko, and before that Shichibukai, pirate to pirate, it's a pirate world, Oda still didn't add any marine as someone Luffy should beat so far, because he won't fight the marines the same way he fights the pirates, pirates are the main target for Luffy, the marines will never be the main target, as even Luffy himself said when he see Aokiji for the first time, the marines are the good guys. But when Revolutionaries involves in it, then the things changes, then you can question who are the good guys? Revolutionaries or marines? Pirates will never be the good guys like them, not even Luffy.
Revolutionaries should handle the marines.
Pirate Luffy can handle the ''evil'' Government, Celestial Dragons, Imu, Gorousei etc.