The problem is there is such a large disparity in player skill these days.
I join pugs for keys from time to time, in the +6-9 range players don’t interrupt, dispel, stun, use CC, they generally just mindlessly DPS and ignore mechanics. They don’t care to learn or even know how to learn what different abilities do.
Compare that to top players learning a new dungeon. You get hit with a debuff, you are reading what it does, can I interrupt it, stun it? sheep it, knockback? Does it have a max range? How does it pick who it goes on? Is it the closest target? Does it prioritize ranged players?
They quickly pick up and process this information and figure out the best way to deal with said mechanics, and the gap widens between “skilled” players, and casual dungeon runners.
Now, there is nothing wrong with running dungeons casually, but what do you really mean when you say you want “hard” content?
If Blizz releases content tuned around better players, a lot of people would complain that it is too hard.
If they release it balanced around the more casual crowd, a large portion of the community would blast through it and consider it “ez mode”.
Mechagon came close in terms of being a good difficulty imo, and I especially had fun doing the hard mode week 1, but it will inevitably be rendered obsolete, and it will be fun to dive back in when it gets split up and added to M+ rotation in 8.25 or 8.3.
I’m not really sure what you are asking for I guess. “Hard” is subjective, and static difficulty dungeons have less replay value.