Nothing has really changed recently for the Job itself, more that the meta-game has shifted due to buffs to Paladin and nerfs to the content's damage check difficulty overall. Because of this shift, the chinks in Dark Knight's defenses and offenses have been more apparent, which is why people are wishing for change. To be completely honest, those chinks have been around since day one, but Paladin was so bad at the start of Heavensward due to the downright hostile raiding environment, that people took Dark Knight out of necessity, not because it was ever "balanced" or well made.