The Wii U will not be a massive success. We need to accept this, you can bet Nintendo has accepted it.
However, people are doomsaying all based on the FY2014 Q1 sales. These sales represent a period of time for which there was no advertising, no marketing, and most importantly, no releases. Yes, these figures are absolutely dire, but the way people are acting, it's as if Nintendo put their all into these three months and it still failed.
Iwata said months ago that they shifted their marketing budget to the second half of the year. We now have at least one first party funded game coming each month in at least one region. We also have a plethora of third party titles finally coming out for the system.
Yes, the PS4 and Xbox One will have an effect on sales this holiday, but the Wii U is the cheaper alternative, and if Nintendo get the marketing just right then the Wii U will have a rebound. It won't suddenly set sales alight, that'd be ridiculous, but the Wii U is not dead and people need to stop acting like it is.
I believe you underestimate their appeal, yes. While the new consoles will make noise, you forget that even though the Wii fad is gone, it opened up the public's eyes to many Nintendo franchises and they will see that there's a new game out and consider purchasing the system.
Super Mario 3D World is the sequel to the fastest selling 3D Mario game of all time, one which with Mario Kart 7 saved the 3DS. It also has local multiplayer for the first ever time. It will appeal to several demographics