I think you are confusing Blizzard's intention. It is true that Blizzard doesn't want another Sunwell where only a small percentage of players has seen the content but the cause is not simply content difficulty.
There are a few reasons that I can think off of the top of my head for why so little saw Sunwell
1. Gating
2. Gear progression was rather difficult
3. Class balance made a rigid raid comp
4. Loosing 1 or 2 tank/heal along the way can often stop progression to the point of disbanding
There may be other reasons but content difficulty is definitely not the sole reason why so little people saw Sunwell. So when Blizzard said they do not want to repeat that mistake again, they do not mean content difficulty alone.
Challenging content is needed for WoW to survive as it is now. Having only easy content will boil entire wow end game into a grind fest for BIS list and I don't see that model working out with a monthly sub fee attached. A prime example of this is SWTOR where people said end game was too easy so they finished all raids and unsubbed (and SWTOR quickly went into F2P mode).
On the other hand, having only hard difficulty for raids also isn't healthy for the game as it discourages new people from going into end game. This is why Blizzard made LFR + normal/heroic mode for every raid + hotfixes to fix overtuned bosses + evetual blanket % nerf so people can remained being properly challenged each step of way.
Harder (a better word imo is challenging) content is definitely needed for WoW to remain as it is.