There are D through A dungeons. Then S. Then Legendary, THEN mythic. But this random vendor from a crappy stall can afford to buy the stuff that drops from a mythic one? That's some shoddy worldbuilding right there.
Why even go through the trouble to establish all of that if all the MC is going to interact with are high level ones, with barely any noticeable difficulty hike from one to the other? It ends up reading less like ideas the author has and more like a checklist works from other authors established for the genre.