Hmm... I don't think how you didn'y notice, but basically whole Alliance army DIED during the blight attack. It is shown from Alliance PoV of Siege, and underlined again when the "remaining" alliance forces meet Nathanos - they have no hope winning against the horde defenders. But, of course, immediately another miracle happens and huge reinforcements of void elves and gnomes arrive in the last possible moment.
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Looking at them after years, they introduced many elements that ultimately lead to complete destruction of original game. During their time however, the consequences of such mechanics weren't obvious - some of them were even liked, and that pulled Blizzard onto the slippery slope that continues to this day. For example:
TBC introduced:
- flying, which later became a main selling point of Cata, and removed interactions between player, environment, and other players
- arenas, while not being bad themselves, lead to Blizzard balancing classes around artificial 3vs3 battles, removing uniqueness, utility, and class identities, while also leading to a serie of more specific problems that plagued PVP from that day, like infinite mana or unkillable healers
- 25 man AND 10 man raids replacing 40 man raids >> the 1st is 50% of reduction in raid size and that might be ok, considering 40 was borderline too much, but 10 is 75% reduction and was closer to ordinary dungeons than true raids. With "positive" reactions to these reductions we later got 10 and 25 man versions of same raid, no loot differences between the version, LFR... do I need to continue?
- and, of course, already with TBC leveling became by design a necesary evil before true content in the endgame, and nothing you did or acquired before max level didn't really matter
WotLK introduced:
- the changes to raids already explained in previous point, but also introduction of normal and heroic versions that needs to be underlined (for a lot of players raids were mainly about experiencing the content, and after fastly completing it on easy mode they felt no drive to do anything more)
- LFG, which removed one of most sought after aspects of WoW Classic
- immense reduction in importance of professions
People were saying it all began to fall down in Cata, but it began to fall much earlier. WotLK had highest total sub numbers, but that number for the first time stagnated. People were already leaving the game en mass. Its just that the games popularity kept pulling a lot of new players in (hence the term Wrath babies) that it equalized the loss of old players. You could be suprised, but whenever I get to know someone that used to play WoW and I ask him when he quit, its almost always WotLK, or early Cata.