No they will make a sequel to super Mario brothers like on Wii with the 4 player co-op. Remember the demo from E3? They will save Mario galaxy 3 if the system needs that single player mario push.
You know, Co-Op didn't even cross my mind... But that seems right! I don't think Nintendo will need to hold on to a Mario solo adventure.. They will release one regardless. Hopefully a staple in every Nintendo console (portable or home) will be a:
Side-scrolling Mario Adventure Free Roam Mario Adventure