Not realistically.
What im stating is the repeatitive cycle of self improvement with strict and harsh demands in terms of improving and knowledge growth.
And the mindset to push said barrier is equal to the mindset applied when raiding difficult content. I.E go again, research more, learn more, gain experience and push the same cycle again and again until you break the content.
I see casual content to be set content with low punishments for mess ups. I think in a casual enviroment. Leaving the PC at any moment should realistically be avaliable, and the punishment for doing so to be rather low.
You talk about ability to prioritise RL over the game. Yet there are parts of HC id argue dont allow u to do this.
You got a child. And that child starts screaming while ur on a dangerous cave, ur basically losing ur char for the sake of attending to ur kid. As a easy example of this.
HC doesnt allow Outside prioritisation, because the punishments are so high for simply doing so. If ur logging in without the garuntee ur not going to be pulled away ur basically throwing ur char to the trash can in multiple circumstances.
Either way. Im pretty much done with HC. 3rd char dead and cant mentally push myself to repeat the cycle, Lol. 
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Era offers the same levelling, same content on a lower punishment system which caters to players who do have RL priorities that simply cant be delayed in circumstance. It allows for mistakes to happen without rampent costs or motivation.
I dont consider HC to be casual content, casual players will play it, but the OP says Retail aint for casuals yet millions of them do play it.
But on the concept of deeming a game “casual” on the bases u “casually play it” is a equal argument to me saying “i aint a casual” and i “i hardcore play it” therefore it cant be.
The game modes hyped. Its hugely popular, and its common for people to jump in. At the end of the day its why we play a mmorpg! To have a living world full of players.
I just dont think the longevity is there for a casual audience.
Players will either reach 60, or burn out trying to reach 60, but being the “casuals game” in my eyes consists of more then just a "levelling experience’
Classic era servers are booming popularity wise, because the quantity of casuals and more whove died and dont wanna restart the process.
That will carry on til the population drops and only the serious will remain realistically.