I played WoW from vanilla up until a year ago, I completed every raid both normal and heroic up until Cho'gall HC in the Twilights raid while the content was the current tier, so yeah I do in fact now what I am talking about.
And thats more or less what heroics boil down too, first spend a few weeks farm the bosses in normal mode to get everyone geared up, then once ready you start doing heroic/hardmode where you fight the exact same looking and sounding boss monster with most of the same fight mechanics bar what I mentioned the extra gimmick tossed in along with more DMG and HP.
And the heroic mode for the most of it drop the exact same gear recoloured with higher ilvl that gains you about 5% more stats per item.
Sure the tactics do change from normal mode to HC, but its still the very same boss.
Ask any long time raider to compare the feeling of first time downing pre nerf Twin emps, Kael'thas, Gruul and others to the feeling the have when downing a firelands boss HC for first time, its not the same at all if you alreayd seen that boss die plenty of times in normal mode and then killing it in harder difficulty as there is seing a hard boss get nailed for the very first time.
Even killing the Lich King first time in ICC just didnt get that feeling cause we'd know we still needed to beat the heroic version, and once the heroic version was defeated we where already sick of it the raid and it felt more of a relief then a joy.
The only boss in recent years to come close was Algalon, being only avaliable in HC as well as being hard and limited to access.
And you seem to ignore that casuals already have their favorite MMOs which is either one of the WoW, SWOTOR etc or are jumping on the newest flavour of the month MMO, aiming purely for casuals without already having a strong ground to do so isn't realy as profitable as you might think, the competition is quite fierce in todays market with even more on the horizon.
And whats wrong with having casual content(ala 5mans in terms of accessibility and difficulty) and raids(more challenging and harder to access), or are you hell bent on having no hardcore in a MMO whatsoever?
I stand behind having both casual and hard content, as long as hard content isnt just recycled fights with trimmed difficulty.
At their implementation the raids pre WotLK was as hard or in some cases harder then the heroic modes of WotLK and forward, so while its not labeld as such by blizzard, easy mode is the default which nowadays is the new standard.
If you'd raided over the course of WoW's lifespan you'd know this.