I can see that the existence of Slayers might wreak havoc with professional sports organisations but unless their demon hunting proclivities come into conflict with the social order prevalent in the Buffyverse it's not obvious why 1800 Slayers would cause more problems than however many vampires and demons there are in this world.

Really interesting issue. I've got the impression that within Jossverse demons' and humans' power structures never mingle. They exist on separate planes. It's just a plot necessity that is and will be maintained as one of the basic rules of the show. But slayers in s8 are depicted as members of human society who are supposed to live by human laws.

Again, I agree that Willow has much greater power than Buffy and in RL The New Initiative would have focused on her, but I believe it's another suspension-of-disbelief moment: since the show's name is Buffy the Vampire Slayer, not Willow the Mighty Witch, Joss is and will be downplaying Willow's power and will focus of slayers' issues.