I have to ask, why did you expect Mario Kart 8 to have a huge launch as opposed to a steady rise and fall? The bolded is what I was more expecting anyways. If anything Mario Kart 8 in the US seems to have actually organically sold to a decent portion of the Wii U fanbase as well as gotten at least some visibility to the console here. Sure in Japan, it didn't seem to have done much, but the traditional gaming industry is having a terrible time in general over there.
1st 2 days of Wii U sales after MK8 were at least 25K, as opposed to the 28K average in June. Could the new 140K new Wii U owners and a good #s of folks who bought Mario Kart 8 now become fairly active users of the console? I think Nintendo needed to invigorate their existing userbase first, and I think MK8 at least gives some hope that's possible. If third parties could sell a good bit of software to a somewhat small userbase, I don't think they would mind that over having a very large, but somewhat inactive userbase. I'm not saying that MK8 sales by themselves imply anything, but it'd be interesting to see whether at least Nintendo can capitalize on this with their first party titles (which unfortunately there is nothing until Hyrule except the mediocre Wii Sports Club).
This is completely anecdotal, but thanks to Mario Kart 8, 3 of my friends bought Wii Us over the weekend. Folks started chatting about it over FB, awareness was built, and thus more friends bought the game to join in on the fun. Being a game you play with others (online too), this could lead to just a general increase in awareness and interest around the console. Sure that could all be gone in 2 days when the MK8 free game deal is over, but we'll see.
Also the 3DS played it way too safe. The experimental titles were out early without any support for traditional titles (Steel Diver, reboot of Pilotwings). Then Nintendo basically had a ton of very traditional but well-polished titles released (Luigi's Mansion 2 and Kid Icarus Uprising are probably some of the more unique ones, and look how well LM2 sold =) ), and now is releasing their smaller known titles (Kirby, Yoshi, etc). More wacky and new titles really needed to be released. Tomodachi, while not new to Japan, is pretty new in the West and I think is the kind of title that might have done well to invigorate the console back in 2012 by bringing in a new/existing audience earlier as opposed to later. It still seems to be doing fairly well so far in Europe from what I see though.