In addition to trilogy-itis, there's something I like to call the "Capcom syndrome".
I haven't played the Castlevania series, but from perpetuated opinions it sounds like C1 kicked ass, C2 fucked up hard, then C3 went back to the kickass, C4 was a remake of 1 that kicked even more ass, then Symphony of The Night on PS1 completely radicalized the franchise, but then C64 came and double-jumped the shark and never got a good title since.
Something similar happened to Devil May Cry, which I have played. 1 kicked ass, 2 fucked up completely, 3 was what 1 should have been but somehow still dropped the ball with the clunky styles. 4 is what the gameplay should have been but fucked up again because it pulled a Metal Gear Solid 2. Now there's that upcoming "Dmc" atrocity that's going to completely kill the franchise dead.