The cornetto trilogy (Shaun of the dead, Hot fuzz and the World's end) has arrested development for main theme.
the world's end is the most blatant example of it.
At the end of
the world's end (ah ah) it is clear that Gary is a new man :
his idealized adolescence's visions have been beheaded (literally)
the place of his past glory has been destroyed (literally)
Gary King (of humans) has found a new sense of self worth by defeating the network
he doesn't rely on childish notions of manhood and self affirmation (alcohol is not needed to be a man, as he orders water)
So it is safe to say that the days obsessed with self indulgent nostalgia are over.
Why choose the kid blanks to team up with?
I am not sure that "why" is the right question to ask.
The ending makes sense/is satisfying because :
it ties nicely with the end of
Shaun of the dead where Shaun chose to hang with the villains of the movie (his zombified roomate)
it ties nicely with the whole premise of the movie (the Golden Mile) but now with a twist (well, two : the blanks and the water) to show the character growth.
it opens on a movie genre that the Cornetto trilogy hasn't touched : western (after zombie, cops and aliens movies). Edit Or, as Nobby points out, Mad Max post-apocalyptic film.
Gary graduated from obnoxious teenager to a man of honor. Instead of using his friends as mere means to have a second go at the Golden Mile, he now takes charge and fight for his mates (the Blanks that got bullied by the bar patrons).