Ghostcrawler discusses how there is a fine line between too easy and making the raid game feel shallow, as people want and expect a challenge but don't want to grow more and more frustrated over time as the tier of raiding winds to an end. The sweet spot, according to GC, is making it feel like every raid group can make some kind of progress.
One very interesting point that Ghostcrawler makes is about encounters nerfing themselves over time, as people learn new strategies, disseminating those strategies amongst the player base, as well as UI mods. Making specific mention of UI mods lends to the overall theme that the UI is moddable for a reason. Also, as we have suspected, Blizzard does like to nerf content slowly over time so that accessibility continues to be the name of the game.