Wii U is an incredibly fun console, and American customers tend to forget that this JAPANESE console isn't only sold in North America! In a wonderful land called Japan, Wii U is the console with the most sold and Xbox and PS4 have rather low sales, but that doesn't mean PS4 and Xbox are failing. It's an international market, but some people are just a tad too narrow minded to see that.
With the release of the new Super Smash Brother's coming out near Christmas I wouldn't be surprised if the number of sales didn't jump up dramatically. The only reason I got a wii was for super Smash brother's Brawl and once I had the system I learned to enjoy it.
That's a weird question because it has already been successful. It may not be as successful as the Wii, but the Wii came out as a unique platform at the time in a very different market. They've sold millions of Wii U consoles since its release. If by success you mean "the best console ever" then you are asking the wrong question.
The Wii U isn't priced cheap enough for casuals and doesn't have sufficient third-party to make the Wii U the console you can have without the PS4 and Xbox One, pretty much leaving it to people who are Nintendo fans. Even with exclusive releases like Mario Kart 8, Pikmen 3, New Super Mario Bros U., and Super Smash Bros, they sold less than it's previous counterparts.
The Gamepad seemed like a cool idea, but the way Nintendo explained and implemented it was very poor or mediocre at best.
With the PS4 alone outselling the Wii U 2:1, I can safely say the Wii U will fail, sad but true. Developers will just skip releasing it's famous titles on it and if someone had a Wii U, it would be treated as the "2nd console".
The Wii U is sure to fail because of the programming. Game developers like simplicity, from transferring one game from one consul to another. Taking one game from X-box 360 to a Wii U is very difficult as you have to make a whole new feature to it, the lower handheld screen. This would take more time and money to complete. Also the price is huge since they included the tablet. The Wii U is sure to fail as it is hard to program for.