Yeah, the NSMBU "killer" (it still sold quite a bit) was NSMB2 because it made NSMBU feel a lot less special and it was also not received very well, which built some bitterness towards the NSMB brand. NSMBU was actually very good and introduced new stuff, but I can't believe they recycled the world themes, the music and the art style.
I hope that after Super Mario 3D World and Donkey Kong Tropical Freeze, the NSMB team feels challenged to do something new and interesting.