There is such a power in FMA only telling Ed and Al's story.
By the end we can see glimpses of all the other people we met throughout the boys' journey, but for most of them the future in uncertain.
In a way it is a very... un-shounen ending. Because while, yes, Edward does get his happy ending (his brother, Winry, and a future in which he can be a happy and attentive father) for everyone else the future is uncertain. Oh, it is certainly hopeful - but it is not written in stone.
We know that Roy got a promotion, but Brigadier General is still three to four ranks away from becoming Führer and it is likely going to take years if it's going to work at all. And while Grumman is definitely not a Homunculus, he is still human - and FMA shows us again and again what horrors humanity is capable of. He's a moderate and on Mustang's side, but in the end we don't know how his and Hawkeye's story ends.
We don't know if they achieve their goal. If they ever end up paying for what they did in Ishval.
We know that Ling ends up becoming Emperor, and we know he has sworn to protect the Chen Clan. We don't know if his goal of creating peace between the warring clans of Xing will bear fruits. We don't know if his reign will be long and prosperous, and we don't know if he will ever find love - he is, after all, a man for whom duty comes first. Lan Fan and Mei are much the same.
We know Scar reclaimed his heritage - maybe even his title as a monk - and that he and Miles are in charge of rebuilding Ishval. We know Mustang has plans to end the occupation of the annexed country. We know there is hope for a people who had every reason to loose it. But we won't ever know what reparations were paid, what troubles were had, what hurdles almost destroyed them.
We know Winry will have friends in the future, and children. We can hope that her automail business becomes a success and that she fulfills all her dreams - we have reason to hope, her happiness is Ed's happiness after all, and we know he would do everything for her.
We see Al travel, and we see him and Ed share ideas and knowledge and dreams. We can imagine a world in which they live long lives and experience more adventures - but at the same time, their story is over.
FMA answers a lot of questions at the end, telling us that for now, everything is okay, our heroes saved the day, their friends (mostly) alive and well.
But it will never give us the ultimate happy end - it will never give us a "and now everyone is happy and everything is good". Oh, it comes close, because it offers us something else, something central to the themes of the story:
The hope that Roy and Riza will succeed, that Ling and Mei and Lan Fan will change the Imperial System, that Scar and Miles will create safety for their people, that Winry will be happy, and Amestris a country worth living for.
We will never know for sure, because it is not their story.
(and thank god for that - the possibility of failure is what makes the hope worth it)
(I'm sure Ed and his automail leg would agree)
