I think it's silly to try to downplay MK8's sales numbers or it's effect on the WiiU's numbers. MK almost always has a 30% attach rate and MK8 is no different. It's still selling and it will continue to keep selling. SSB is just one more great reason to buy the system.
XBone is going to get a lot of great games this Xmas, but most of them will also be on the PS4. Most people who want those games will likely pick up a PS4 - not an XBone. I think the WiiU is going to be a great companion system as it's game lineup is vastly different and it doesn't compete directly with either system.