The Wii was a huge success but a huge disappointment. So many people bought it and only liked the games packaged with it and a few other titles like Wii Fit and Just dance.
Nintendo lost a huge portion of their core base and that's why the Wii U sales are so poor.
Nintendo is in a really awkward position. Many suggest they become solely a game production company. The problem with that is they'd have to produce 'modern games' which cost more to produce and which is a huge concern of Nintendo's and has been one of the causes for delays in Wii U software which is already a bit dated in software compared to what will be releasing on PS4 and XB1.
Others suggest Nintendo make a truly modern console. This is nearly impossible considering the huge losses the XB1 and PS4 are sold at. Nintendo doesn't have the safety net of a parent company like M$ and $ony. Also the last time they released a competitive console, the Gamecube, it sold poorly. The Gamecube had third party support, modern graphics, a popular controller and Nintendo's first party titles and it was a disappointment.
Nintendo has to expand their software production somehow. They have to buy studios, release slightly lower quality products (so they have a larger selection), and they have to combine their 3DS and Wii U business. The 3DS has a much different OS, it doesn't support Miiverse, you have to buy the same VC titles for Wii U and 3DS, you can't share the two.
All this seems so obvious but Nintendo maintains the status quo.