It's not so much a decline, but rather a change: they made the game more intuitive to new and returning players. This has pros and cons.
  • The pros are that a new player understands what cast is a tank buster, a cleave, a stack, etc. Without this, these players would, in many cases, never learn what these casts do. It's one thing to not know at first and another to never know, and they just never seemed to know the mechanics. It feels so much cleaner now.
  • The cons are that knowing this right away makes it easier and reduces the failure rate. Although they have tried to address that in the newer dungeon designs, the issue has become lack of RNG or lack of dungeon variety overall.
With rotations, there were similar issues. Players would sometimes never figure out how to do their rotation right, and do significantly less damage as a result (easy example could be summoning Titan all the time on SMN). There were good parts and bad parts to their method of simplifying it:
  • The good parts were things like making lone GCDs such as ranged attacks not interrupt melee combos, making abilities like Flourish not overwrite all your procs, which were just annoying things more than anything.
  • The bad parts were removing the ability to get rewarded for playing well, such as increased damage on abilities for having both DoTs up on BRD, the cones on WAR and SAM, cleric stance, although DRK was admittedly over the top for most players they could have kept a lot of the interaction between abilities on DRK to reward using it situationally, the healer mattering in regular content, etc.
I actually think the Extreme, Criterion, Chaotic and Savage raids they make are good, the problem is really that there are a lot of people that want something between Dungeons and Extremes that can be direct queued for and will be cleared sometime within the timer, or that is similar in difficulty to Eureka NMs and Bozja CEs, and I think SE just doesn't understand that. And without a substantial amount content of that difficulty level, those players feel there is nothing to do because it's the only content they would ever be interested in doing.