They're a retcon introduced in what, Warcraft 3? But whatever.
Stop talking about "filler", your stupid anime terminology makes no sense in relation to WoW. Basically your argument is saying that unless every piece of lore is about the Titans it sucks. Hate to break it to you but Warcraft is about the mortal races, the Titans are just background material. And if we explored Titan lore we'd wind up killing Sargeras in one expansion and that'd be the end of the entire Warcraft franchise - which is another reason why they're never going to take centre stage.