Thinking back over this issue, I'm now inclined to think of what Willow actually did as no spell at all. An actual spell would have been selectively empowering the Potentials there in Sunnydale while keeping all of the other Potentials in the world totally unaffected and normal. Instead of taking the time to configure such a spell, Willow just figuratively tossed a lighted match into a powder keg, activating every girl indiscriminately, with no control or focus whatsoever. What she did might have been the actual reason why no more Slayers were called before Fray's time. The energy, for lack of a better word, that activated the Calling was so depleted in that one instant that it took centuries to renew, recharge, or replenish itself, resulting in what might very well be a temporary cessation of any more new Slayers being Called.
No not at all.
Willow spell doesn't seem to be a new spell, all it did was bend the rules of the last Slayer spell, removing the 'only one' clause.
That had no effect what so ever on the amount of slayer energy out there, seeing how the Slayer power doesn't move from one girl to the next, each girl has the power it seems to be an unlimited source. New slayers are activated in season 8, so the process wasn't stopped becasue of Willow. She didn't drain the slayer energy, it was still in effect, just with a new set of rules. The activation spell is not to blame for the no slayers in Fray.
They haven't made it clear in season 9 whats happened now. The Slayers got to keep their powers cuz they were inside them, and potentials have the power inside them, but presumably the spell that activates them once they come of age was no longer in effect, now that magic is back we could see something new in regards to new slayers.
Remember, Joss verified that the battle with Twilight in Season 8 was, in fact, the battle shown in "Fray" that expelled all of the demons and vampires from Earth.
Has he? I'm 99% sure he hasn't, you mentioned this before and when you were asked for a source, you said it was your opinion, but Whedon hasn't actually confirmed it.
Since Buffy changed the outcome of that battle, demons and vampires weren't expelled, so the Slayers are still needed.
That was never the outcome of Twilight, it's the opposite, all the good people go to Twilight, all the vampires stay on Buffys world, the plans are very similar but they dont actually match up in a lot of ways.
There is a lot of intentional mistakes in Fray, historical inaccuracies, like how it's named the battle of Starbucks and such, but had Twilight been successful, Buffy and Angel would have left earth, they would have pulled across some of the population, but wouldn't have been able to save all and they would have been in a new universe, with no demons. Maybe if this version were true, it was one of those historical errors and rather then the demons and magic being pulled out, they were left behind and Fray is actually the future of the Twilight universe.
But then there are some problems with that, firstly how would Buffy have travelled there in season 8, unless it wasn't just time travel but jumped dimensions also?
Also Fray mentions the "last great Watcher Gates" who sacrificed himself, which could be seen as Giles death, but with the historical errors. But in order for Giles to die, Twilight doesn't go ahead. I guess you could make it work, by saying that After Buffy and Angel reject Twilight, Twilight takes over Angel, kills Giles and before Buffy destroys the Seed, Twilight drags Buffy into the new universe, it also takes much of the population with it (would it?), and seals off the old universe with all evil behind it. And Frays time just has it mixed up about which who went though the portal?
But even so, it doesn't actually line up, Frays time line is a mixture of both the Twilight portal scenario and the Seed being destroyed ending magic, and the two are juxtaposing scenarios, they were never something that mixed.
Also the comics suggest Frays time is still the future. Fray wanted to stop Buffy going back to the past, so their world wouldn't change, but Buffy does go back and things dont change, meaning everything Buffy has done is still heading towards Frays time.
In Fray's reality, no Slayer was Called before her because none was needed. Now, however, they will be, and there just might not be any new Potentials created, meaning that the existing Slayers are in effect, the last of their kind for centuries to come.
Willow just might have succeeded where the First Evil failed in destroying the Slayer Line.
Like I said, Willow didn't drain the energy, new slayers were still activated, this theory is canonly already disproved, presumably had the Seed not been destroyed they would have continued to be activated.