Anon, I want to be really clear here and say that you're not "bad" for what you posted. Really, I'm not trying to say you're dumb or anything. Are you wrong? Kinda, but that's okay. In some ways this series is pretty straightforward, and in some ways it's kind of nuanced and complex. Unless you talk about the show and the things you like and dislike, or things you understand or don't, you'll never gain a deeper understanding of anything. You can see this in the show itself when Kuu leaves and Rakka falls into a depression. She can't wrap her head around why Haibane leave or why they even need to leave, it's only when she talks with the others that she can understand and eventually accept that it's just a fact of life.
The story we get with Reki and Rakka present is just a microcosm of the existence of the Haibane and Glie as a whole. It's impossible to say how many Haibane have left before them, how long Glie has been there, how many Haibane will come and go after everyone at Old Home has taken flight. In some ways I guess you could call the show a slice-of-life, because we are given a small window into life in this strange town for a few months, and we get to see what drama develops. You may have found the stories of Reki and Rakka touching and compelling, but think of all the others who have gone before them. How much suffering and happiness has been expressed in the walls there at Old Home. How many other Haibane have woken up to find friends gone, how many Haibane have layed awake at night wondering when their time to leave will come.
It's understandable to care about Reki and Rakka. They're great characters who are compelling and fun to think about. But you can't let yourself focus solely on them, because the series isn't just about them. Just as Reki and Kuu left, so will the others. And just as Rakka came into that world, so will many more. Even at the end there's already 2 more Haibane ready and waiting to be born and experience Glie.