I remember when I had a conversation with a particular member - it may have been Rol - about when exactly SM3DL would be passed up by MK7. I believe the notion was that it would happen rapidly, as 3DL was not a real Mario game.
3DL has managed to hold onto its weekly lead since launch, I think it will probably end up crossing 10 million first. MK7 will probably beat it to 15, though.