There’s a subtle underlying trend in Book 3’s finale that seems to get overlooked due to the fact that so many people other than Korra are responsible for the victory over the Red Lotus — almost everything that happens is at least an indirect result of Korra’s actions.
Think about Korra’s necessary allies for a moment: Mako, Bolin, and Asami; Tonraq; Lin, Suyin, and the Metal Clan; Kai; Jinora and the airbenders. Every single one of them is someone who Korra helped grow into the sort of person who could do what needed to be done to achieve victory. Team Avatar might have the weakest ties, but even there, Korra helped Mako rediscover his inner stability (lightning), Bolin discover his confidence (lavabending), and Asami grow from an acquaintance into a close confidant.
The other characters grew in even more obvious ways.
Tonraq learned to trust Korra, and he proved vital on Laghima’s Peak because he was able to fight alongside her as an equal.
Lin and Su’s reconciliation — which probably wouldn’t have happened if Korra hadn’t pushed the issue, and definitely wouldn’t have happened if Opal wasn’t an airbender — allowed them to work together to take down P’li.
Korra’s own relationship with Su meant that she had a whole bunch of Metal Clan allies, one of whom ended up saving Tonraq’s life.
Kai went from a selfish thief to someone completely willing to throw away his own life to allow the airbenders to escape because of Korra’s offer of unearned trust, and his attempted sacrifice put him in a position to lead the rest of her allies to where they needed to go to help her.
Jinora’s growth was largely dependent on her relationship with Korra in Book 2, and she wouldn’t have even been around had Korra not done everything in her power to save her from Unalaq.
The majority of the airbenders were people who Korra had freed from the Earth Queen’s captivity. Even Opal was freed from her fear of disappointing her mother by leaving due to the effects of Korra’s visit to the Metal Clan.
In other words, the Book 3 finale is one giant exploration of the idea Iroh introduced in A New Spiritual Age — “Sometimes the best way to solve your own problems is to help someone else.” (This would make an interesting gifset, actually) Korra might not have been able to defeat the Red Lotus on her own, but the good that she did for others ended up being the key to the Red Lotus’ defeat.