This is the single most biased piece of work I've ever read on MMOC.
No context, just grating and almost whining in pointing to things that cannot be taken at face value. I'm genuinely saddened that people will read through your post and think themselves more informed about the state of raiding since Wrath of the Lich King.
In the interest of making sure this is a constructive post, at the absolute least take out your abysmal commentary and instead add two things:
1) Graphs for middle tiers.
2) Explanations on each individual tier and the exact state of the game at the time of progression.
If the second point seems daunting, that's because it is. Challenging 5 man content in early Cataclysm and rather inaccessible raids early-on pulled an astronomical majority of players away from playing that facerolled through WotLK. Pugs that could be found at all times of the day were now scrapped in favor of guild perks. And in the final tier, LFR was added. No matter how you slice it, this did pull players away from doing normal modes, and still does. The people that cite scheduling issues have perfectly valid complaints, but even they are not immune to the simplicity that LFR offers. LFR offers laziness, where normal modes do not. Pandaria has had LFR throughout, WotLK and Cata did not.
I think for this to be a truly valid discussion about Pandaria, one would also need the number of kills being represented at the LFR level. To represent WotLK and Cataclysm, the middle tiers are necessary. Ulduar was not a shining peak for the casual player, and neither was Firelands.
Yogg 0-Light and Heroic LK were two of the most complex and difficult encounters ever created, and they stand in that same triumphant state even today, after two expansions of new bosses. Heroic Ragnaros and Heroic Lei Shen are the only two bosses to enter even the same wavelength. I would not say that the complexity of encounters has truly increased that much at the highest end. On a lower end level, is Jinrokh really much harder than Marrowgar was? Stone Guards to Northrend Beasts? If anything has changed, it would be the flavor of the encounters. BC was by and large a notorious culprit for having some of the most inaccessible raid content ever put out- but look at a boss like Mag'theridon. Incredibly straightforward, yet an average pug found him to be an incredibly difficult task. Kill adds, click the boxes when you are supposed to. Flavor.
Lastly, I'd like to remind the community that this game was a success even when casual players were given nothing, as in the Burning Crusade. It is on the backs of the raiding community and Blizzard's love for stellar raid content that this game exists in such high standing today. Do not be so callous as to forget that.