First of all, lets agree all that casual is not a bad player. Is a player that cannot dedicate to a guild schedule or that he does not care to min - maxing. Of course this is very generalized but in order to avoid debate on what casuals are, lets just talk about people who don't play this game for the raid part (which are the majority with blizzard statistics).
Also lets agree that if 1-2% of the content is super hard and the 98% is super easy, we cannot call the game "hard". Easier game means content is consumed too fast and developers need to release content even faster. Everything in the game is trivialy easy except the normal and heroic raid.
For those that don't raid there is not any feeling of accomplishment. Back in Vanilla/TBC you felt accomplishment for reaching the max level. You felt accomplishment for completing a group quest, for completing a difficult dungeon. Even if you didn't raid, there were many things in the world to do that made you feel you accomplish something.
Now, unless you are part of a guild that does normal/heroic raid you don't feel that you accomplish anything, you just take part in a lower gear tredmill via easy and boring dailies and easy/boring LFR. Unique class quests? no, that will cost a raid tier. more features? no, that would cost a raid tier...everything we ask is just not a priority because Blizzard needs to release the new raid in 3-4 months.
Back in vanilla/tbc there were not any threads about "when the new raid will release?" or "wtf blizzard? 6 months without new raids?". All the game development focus now is just to feed the raiding community (and LFR also). Everything that done to the game the last years is to ensure that players will start raiding as fast as possible. But what type of players are only interested to raid as fast as possible? Is it the casuals?
In conclusion, I don't think that Blizzard develop the game for the casuals but to ensure that the Raiders will raid as fast as possible and they will have a new raid every 3-4 months (except the period before expansion that they focus on new xpac). Casuals did not asked for trivial leveling or heirlooms. They never asked to remove the group quests and elite zones from the world and unique class quests in order to get more raids..