Why can't the assumption be that 2D Mario is awesome, 3D Mario is awesome, so a hybrid Mario would be awesome. I'm sure we could argue whether 3D Mario is more complex, but platforming is platforming. No Mario has ever been as complex as 80% of the games released now-a-days.
Plus, I don't know what you are looking for anymore. They release a hybrid Mario that many people love and gets almost all 9-10 reviews and you say they dumb it down. If they'd have released a direct sequel to M64 you would say they've ran out of ideas and just release the same old crap you played years ago. What would make you happy? A gory FPS like everyone else comes out with? If you know someone with a Wii U try the game before you dismiss it.
M64 was amazing and worth all the 10's it got for the time it was released and I'm sure Nintendo will go back to that style at some point. 3D World impresses me with the way it changes up the level design and really keeps things interesting. I think the cat suit is the best powerup since the cape. They can do alot with the fixed camera perspective. Sometimes a level is more like M64 and other times it is more like NSMB. They put alot of work into this title. The fixed perspective leads to better platforming than M64 and allows them to hide items better in my opinion. I like this title. They might make 1 more or they might move on but I'm glad they made this. And I'm glad I've bought a Wii U even if it doesn't get all the games the other consoles get.