You guys know the alternative Chosen script was bogus, right?
It was fanfic. Even if it's connected to the book, it doesn't matter. The books are just professional fanfic. Joss disavows any connection to them, other than monetary.
The whole connection with Dawn to the slayer line thing comes from the so-called alternative script. It's a fanfic idea, that has nothing to do with the reality of the Buffyverse as Joss and company have presented it.
As far as what happens after the potentials die, that's explained in Fray. Fray is Buffyverse, because Joss Whedon wrote it.
This is what happens. There is a giant battle in which a slayer, perhaps aided by mystical allies banishes all demons and magiks from this reality. After that girls still appear with the power, but they are never "chosen", meaning they are never given a watcher and trained, or shown where to direct their power. They have nothing to fight. The watchers degenerate into a cult of madmen.
At the time of Fray a vampire army is forming with the intention of re-opening the connection between this reality and the demon one. This is why Fray is called. The watchers however have gone nuts, so a demon is sent to train, and direct her towards the battle against supernatural evil.
Joss has promised to explain all this better when he writes the next Fray.