I feel like Eureka/Bozja are good point to define what is midcore : as much as doing it once for the story (and maybe one relic) is pretty much casual, farming it for month to get several/all of them is definitely midcore. I dare you to pretend a player that got all the SB relics is a casual player.
That doesn't mean Extreme cannot be considered as midcore but limiting it to difficulty is wrong. In the end this whole "lack of midcore content" really was a "lack of content with shelf-life content", and it became more and more obvious as the time went. I give SE credit for trying to answer the complaint, but I hope that, with the criticism about the Chaotic raid (or everything surrounding the fight itself, that's pretty well recieved by those it's aimed at), they'll realise they missed the point of the criticism. Chaotic seem to be a really good fight for those who enjoy that, but I feel it's not the right answer to the "nothing to do/lack of midcore content" complaints.