We have midcore content... just too little too late.

[ReynTime]

Content that is cleared in one run 99% of the time can't be considered "mid" core.
And content that takes a week or longer to be cleared with a static group can't be considered "mid" either.

It makes sense to classify midcore as content that has a fair chance of being cleared in one run, but has a high risk of leading into multiple wipes, or the run ending up in failure with randoms but not necessarily requiring much practice.
The only things that fall into that are Extreme trials, the instances within instances (BA, DR, Zadnor, etc), deep dungeon full clear and Unreals. Variant dungeons are significantly easier than that. While Criterion are already close to Savage in difficulty. They had the chance to make Criterion midcore, and they blew it. So have Savage and Savage-Savage instead. *slowclap*
But honestly if you enjoy deep dungeons there isn't exactly a complete lack of midcore content. If you don't... it's really dry.