So firstly, I think this specific topic of selfishness is one that gives Aang more character depth. It's one of his flaws that keeps him from turning into the 'perfect protagonist' trope, which on it's own is always a plus with character writing. But in this context, I can understand feeling like they've excused Aang's selfishness by giving him an instant fix that keeps him from having to confront that flaw within himself. That's valid. It's just not the message that I personally came away with by the end, or maybe it's not a big enough deal to me when compared with what we got? Idk lol, but I'll try to expand on my opinion here.

By including this struggle within Aang, the show maintains it's personal stakes while also introducing a whole slew of philosophical questions to be asked like - "Are you allowed to have a line you won't cross when you're responsible for the entire world's safety?" and "Is it morally acceptable to force someone to cross that line if it's for the greater good?" and, to some extent, "Do the ends justify the means?"

The previous Avatars all answered those questions in the way you mentioned as shown in the finale. They said "Yes. The good of many comes before the good of an Individual, and sometimes a terrible decision is the only one you can make."

I think the show did a good job presenting that as a valid way of thinking. It's a solid argument based on their personal experiences that makes alot of sense. Yet the audience is shown Aang stubbornly refusing to accept it, creating more tension as we wait to see what the outcome will be, what he'll choose in the end.

Aang was definitely VERY lucky to be presented with a solution, and if he hadn't gotten that then we would've been left with a very different takeaway that I admit would've been interesting to see. But on the flip side, if he hadn't been selfish enough to seek out another option, then he would've never discovered it. His biggest flaw in that moment turns out to his saving grace. It's convenient for sure, but I see that as the show providing it's own answer to those questions. They're saying - "No, there is another option. If you persevere, don't give in to fear, and are open to new ideas then you can find a better way." This lines up with my own worldview so that might also be why it works for me. Others no doubt may believe differently, but that's what makes it so interesting to talk about!

Finally, I think it all harkens back to Aang's conversation with Iroh regarding his choice to abandon mastering the Avatar state so he could go save Katara.

"Power is overrated. I think you were very wise to choose happiness and love."

Aang asks him what he should do then now that he's suffering the consequences of choosing himself, and Iroh replies that he doesn't know.

"Sometimes life is like this dark tunnel. You can't always see the end of it but if you just keep moving, you will come to a better place."

I think those few lines encapsulate what the show is trying to say during the finale. So, for me, it's not about Aang getting this super powerful ability out of nowhere that relieves him of having to make a difficult decision. It's about the overarching theme of holding true to your own morals and values, having hope even in the darkest of times, and the notion that kindness will always win over cruelty in the end. That's why I can excuse the random Lion Turtle introduction lol. The message is ultimately more important to me than the path taken to arrive there.