IMO, people missuse Hardcore.
Hardcore is not difficulty, necessarilly.
Hardcore is time investment.
Mythic Azshara is a tough boss no matter what. Taking a raid in there and killing her is hard.
Pulling her 500 times in 1 lockout is hardcore.
Pulling her 500 times across 3 months is casual.
The difficulty of the content doesnt change (independent of your ilvl slowly going up).
Hardcore - Casual spectrum is about time investment, not talent.
Good - Bad spectrum is the players ability to play and adapt.
Hard - Easy is the difficulty of the content either in gear required or mechanical skill involved to dodge the bad and kill the bad guy before he kills you.
This is how you have hardcore guilds with a spectrum of good and bad players looking to dominate easy content with speed running in vanilla.