We have now cleared all misconceptions about how DH and warlock share the same lore and motivations, now we have only the same magic school and general "theme". Priests and paladins share the same magic school and general theme, light. Still paladin and priest works as different classes. Yeah, priest does have shadow spec but no one is complaining about how it overlaps with warlocks shadow magic and shadow priest is even caster.
Also shadow priest was largely invention of WoW, even though WC3 does have shadowpriest troll.