Five years is not 'old'. The fact is that LK changed a lot of fundamentals about how to play the game. AOE DPS was key all of a sudden and you needed to be capable of if (it's when rogues got FoK and shadow priests got Mind Sear). Many secondary class abilities were rendered irrelevant because tanks could hold a lot of mobs, healers could keep them up and DPS could AOE so CC was not needed either prior to pulls or in combat.
There's nothing objectively WRONG with that, but it was a very different playstyle than TBC and Vanilla and you saw a lot of exits which balanced out new players.