Our artifacts are blessed/infused by council, I guess lorewise that great power comes from that. I mean before that you could say they were just very nicely crafted weapons.
Sargeras dies when council sacrifices itself/their power, most likely illidan somehow working as focusing iris or something. I mean he is powerful mage still, those skill don't just vanish?
Most likely that power is too much for illidans body and we get drama how he says last word blabla sacrifice, tyrande cryes and malfurion screams brother.
Demon hunters then join army of light and start hunting all demons and be forgotten in next expansion, unless blizzard lets wow die with some dignity.
Next expansion is about against void and anduin is big cahoot, now instead mages (khadgar,dalaran,nightborne etc.) priest and holy-users are in big role. Or I assume holy is opposite of void?