Tbh, Mythic0 being the old heroic, where does this exactly come from? I mean looking at the difficulty i think we can easily state that while legion mythics are not hard, they are definately harder then WoD heroic (which were more on par with Legion heroics in difficulty), they are harder then MoP heroics were, Cata heroics were harder (but only pre nerf), and they are deffo harder than Wrath Heroics, and on par, or maybe even harder then WoD Mythics (which were pretty easy also), which were never on the LFD tool at all.
If i would have to rank them i would say
Legion M+ = WoD CM diff (M+ 2-3 is prolly easier then WoD CM Silver, M+10 or higher is prolly harder then WoD CM Gold, atleast thats my feeling)
Legion Mythic = Wod Mythic (or a tad harder, hard to judge since WoD Mythics came later in the progression cycle)
Legion Herioc = WoD Heroic / MoP heroic (altough MoP ones were prolly easier)
Legion Normal (all in party 110) = WoD normal lvl 100 dungeon
Legion Normal (not all 110) = WoD levelening dungeon
I don't really see the bullying either, i just see 2 separate progression path's for 5 man groups, which both give the gear needed to get into organised raiding if you would want that.
One trough LFD going from normal > Heroic > LFR (to see the full game story) After this you can stop up to Normal raiding if you would like.
The other being Mythic > Mythic + (content for organised groups of players) which can be used as a step up to Normal (can be skipped when taking this route quite easy) or Heroic raiding