To me, it seems dissatisfaction with the lore has been steadily growing a direct reversal from the incremental increases it got with the RTS series.
Question is, can anyone pinpoint when the downtown actually started and what was the cause?
Personally I feel the popularity of Warcraft has continued because of it's game playability, not because of its story, where the story was an asset and major attraction that spiced up the earlier series, it's often been a source of disappointment and let down - as to why? I'm not sure, but it has to be developments failure to take it as seriously as they do art, music and gameplay - which I feel was a major blunder.
As for when it went downhill, i'm not so sure, I think the decision to make world of Warcraft continue the story of the franchise in TBC was the major turning point as the way they presented the game (i.e. it's format) was not the best to telling a good story, and they certainly didn't make the changes they needed to pull that part of the game off.
Classic wow wasn't a story driven game, it was more sandbox, stories were local to zones and peoples for the most part as you were exploring the world of the RTS series that major events had happened. Departing from this is in TBC is where the error was made or refusing to seriously put effort into the main storyline - wow in TBC continued the zone stories, but the main storylines could never be brought out properly with the power they needed. The raids which were turned into the major focus points of the story themselves told no story where I feel if they had made raids into massive story telling events that pulled together the clues contained in the levelling zone quests, it could have worked as an alternative to an SWToR full on approach. But they never did, showing their dis-regard, and I feel it was the main thing.
I think other key things that hurt the story happened when they made wow a 2 faction game instead of the 4 or 5 factions WC3 had (Alliance, Horde, Night elves, Undead and Illidari by the end of TFT), in wow they also eroded a lot of unique touches, like alliance warlocks, tauren druids (incl female druids and male NE priests), and also by making every thing in the game canon, things became ridiculous (hearthstones for example actually being a thing, and dying more or less becoming meaningless) - however these are smaller issues that better story development could have utilised to much greater effect or gotten around.
Eager to hear what you think was the cause, and when you reckon it happened. I know some thing WotLK was the height of wow story telling, but it wasn't, it had many lame things about it, and the rot had already long started in my opnion. Legion imo had the best story telling for an expansion followed by MoP - in the way they told it in the game (not necessarily the quality of the story and it's plot although I also feel Legion wins that too). Still they are all patch jobs for a bad direction that started in TBC.
If I were to do it again, I would have developed an actual RTS or single player game to tell TBC/WotLK and it would have been very different then introduced the expanded realms to World of Warcraft where you could explore the zones in the aftermath of the events yo had experienced. I would tie hand in hand, new single player or RTS game with an expansion.