MK8 was also the top selling SKU for a lot of June too, so more points to Amazon!
Regarding Wii U, it's obvious that it'll drop, but with Hyrule Warriors and Bayonetta coming I can see sales not being that bad. I know those two seem a bit niche, but at the very least they are types of games you can barely find on Wii U now, so they might attract new people. I mean, Pikmin 3 sold 100k copies on its first month so those two have a chance, with Hyrule Warriors being linked to Zelda and Bayonetta 1 being a bit of a cult classic.
For the future, Smash will obviously help and it'll be very interesting to see if Amiibo has any effect on Wii U sales. I'm very curious about Amiibo too, because if it catches on, I can see it having a nice effect on less popular titles (for example, the Captain Toad game). Nintendo is trying to make the Wii U appealing without killing themselves in the process and it'll be very interesting to see how that pans out.