Do you know why Oda creates problems? So that Luffy can once again be portrayed as a hero.
The RA should solve this problem, but the RA aren't the main characters. Their victory doesn't add to the story, but takes away from it. Therefore, Luffy will be the one to save Sabo (if he needs saving) as that furthers the story and his character.
The RA's goal is essentially to bring about a new government by destroying the current WG. They won't succeed without Luffy's help.
One thing that this story does is give Luffy reason to fight for things he generally doesn't care about. For example:
- Luffy didn't care about Alabasta, but because his friend. Vivi cares about Alabasta, he intervened.
- Luffy didn't care about Fishman Island's problem, he only intervened because Hody took his friends hostage and through that, he saved the country.
- Luffy didn't care about Dressrosa initially. All he cared about was Rebecca fed him, Joker made her cry and therefore, he'll defeat Joker; Joker also hurt Law, and because of that, he defeats Joker.
Why is Luffy saving Wano? Not because he wanted to. For the most part, it's because of Momonsuke asking him to do so as a friend and because of O-Tama and because of Law allying with him.
Luffy's friends and loved ones take him to the problems he needs to solve. Sabo will likely be the reason he defeats the WG. By defeeating the WG, he more or less achieves what the RA want and more or less furthers his identity as a hero.
Luffy's motivated by 2 things: Friends (Loved ones) and Adventure. If his friends are in trouble, he'll save them and if he's being led into an adventure, he's all in.