I don't see why they couldn't.
There are plenty of existing dungeons MSQ ones at that that are the difficulty I would expect.
Mt. Gulg? While yeah there is less forced breaks between packs of mobs (you can pull 3 for instance), the trash actually hits hard for once. They don't want people to pull wall to wall constantly? Make trash more threatening. While I think Boss #2 is a snooze, 1 and 3 are fine. Amaurot has trash that is likewise more threatening. If people want to wall to wall they can, but it won't be as easy as it is currently.
Another thing? Unavoidable damage can be more frequent and less predictable. Why don't we have more bosses that will randomly hit a player that isn't the tank? Why don't we have raid wide damage we can't avoid under every 60s? Have you SEEN a healer's tool kit? They have no less than 4 answers to damage and most of them ARE up every 60s. A raid wide every 30 isn't going to kill them.
Tankbusters can also be more frequent. Hell, the bosses can just auto attack harder than they do and actually may want WHM or AST to pop a Regen/A. Benefic.
Wind ups can be shortened. I don't need a telegraph lasting for 8s when 5 or 6 is just fine.
It is not that hard to make dungeons more challenging and not be brutal.