I do not see hardcore and casual being difficulty related. I see it as being how long it takes to reach the rewards at the end.

Casual and Hardcores both want challanging content. Most hardcore I know from XI and other mmos want content that is longer lived, takes longer to accomplish, and has greater rewards for the time spent. Casual content is the 30min-1hr dungeons/raids with rewards at the end of them. By design hardcore content is extremely restrictive to casuals because if it takes a hardcore player 1 month to get it takes a casual 5-6months to get. This is why you get the WoW is a casual game start to finish argument. WoW has some really challenging content on hard mode, especially without mods, but none of it is true hardcore content. It is still a 1hr raid with a chance of epic gear out of 1 run.

Hardcore: Long term goal
Casual: Short term goal (As in it does not take a large amount of time to accomplish)

I know the definition of hardcore and casual as drastically changed but if you think about it when you define a casual player it is usally one who does not have alot of time to dedicate. So hardcore is one who has alot of time to dedicate. I am not really sure where the definition change happen to where hardcore = hard content, and casual = easy content.

Getting the best gear in the game by completing a short term goal is a handout, little time dedicated for huge rewards. Even relics in XIV I would say are 4/10 on the hardcore scale. I have people in my linkshell that got one from start to finish in a week. Just have to have a really solid group to plow through the content.