Several problems with this idea. I'll just give some examples.
Around the time of MoP's release, the novel Tides of War was released. It, of course, covered the events leading up to, during, and after the destruction of Theramore. In-game, there was nothing covering this, except the aftermath in the form of a scenario as the pre-event for MoP. Most players were confused why Theramore was destroyed, since we didn't see any of the event until after Theramore was already destroyed.
MoP also saw the Vol'jin novel Shadows of the Horde. Because so many people were pissed that the story was left out of the game, that book wound up covering almost nothing of consequence - we saw the gist of the novel's consequences already in-game. The novel was released in July 2013, and it covers events that happened in November 2012 (patch 5.1). It left players not caring about it, seeing it as a useless reason to spend money to see lore that was already mostly seen in-game. Of course, the novel covers things that we don't see in-game (like Vol'jin studying with the Shado-Pan, and their showdown with the Mogu and Zandalari), but we had already seen Vol'jin alive and well in-game, leaving the novel entirely useless. People were instead pissed off that they didn't get to see the battle between the Shado-Pan and the Mogu/Zandalari, and instead were shown Vol'jin being stabbed, and Vol'jin being fine. It wasn't balanced well with the story in-game at all.
Story needs to be in a role playing game. How would WoW continue to exist without quests, raids, scenarios, events, books, pet text, and all other things that provide various levels of story? People have asked for more lore in-game for many years, not less. Making an entirely different game to see the story that should be told to us in the game people are already playing would only make people angry, calling Blizzard greedy, lazy, and out-of-touch.