In any Metroidvania game, there are two stages of play:
Stage 1: Progressing bit-by-bit, exploring the castle for the first time, watching the story, fighting while underlevelled.
Stage 2: After beating the game once, dicking around and destroying things until you've hit level 99, collected stuff, and are bored with all the random weapons.
Harmony of Despair is simply Stage 2.  It cuts to the chase - it's like Order of Ecclesia once you unlock Hard Mode l. 255.  By that time, you've beaten OOE at least twice (more likely three or four times), so the plot is secondary to becoming a Goddess of Destruction.
Also, I have no idea why people seem to have expected the game to be filled with new content.  It was always marketed as a copy-pasted mishmash of stuff from older games, with new sprites only if it was necessary for brand-new attacks.  The POINT is that you can walk through a section of, say, SOTN and beat a boss there, or bounce around Castlevania 1.  Complaining about reused sprites has a place in many games, but this one?  New sprites were never the purpose of the game (although there is some new stuff,mostly in a couple of backgrounds).  It never advertised itself as such.  Complaining about the reuse in HOD is like complaining that Belmonts use whips when you hoped they would have gatling guns.