I think even JW eventually figured out that he'd screwed up with that spell. Witness Dana, the superpowered psycho murderess in Damage. Yay Buffy and Willow, I'm sure her victims were thrilled by the Girl Power. And note that when Angel proposed the Big Battle to his fellow FGers in Power Play, he was meticulous in spelling out that they were probably doomed and he couldn't order them to do it. It's nice that JW finally figured out what the word choice means. It's too bad he had to ruin the BtVS series finale first.
How do you know that JW "eventually figured out that he'd screwed up with that spell"? How do you if that is exactly how he felt? What if he knew that the spell wouldn't be perfect? The Shadowmen's spell didn't turn out to be so perfect . . . as Faith becoming a rogue Slayer would attest. How do you know that Faith wasn't the first rogue Slayer?
All of the Slayers have been victims since the moment they were born, when they became Potentials - thanks to the Shadowmen's magic.
Is this really about Buffy and Willow changing the Slayer line? Or is this about many young girls with powerful strength? And how do you know that all of them had assumed Slayer duties? How do you know that Dana would have NEVER become a Slayer, if Willow had not cast her spell? What if Faith had died in the near future . . . and Dana became THE Slayer?
Here is an interesting article on Willow called "A Broken Yellow Crayon":
http://www.zachsmind.com/Buffy/002BuffyCrayon.html