Wii U will do well compared to how it's been doing which if we're being honest, isn't that difficult to do. There will be enough total first party titles on Wii U where people can now look at it as a worthwhile purchase, with Mario Kart 8 and Super Smash Bros. being the biggest draws. In May, there may have been people who were intrigued by Wii U because of Mario Kart 8 yet remained on the fence. The addition of Super Smash Bros. could be enough for them to make the jump, especially during the holiday season when people are willing to spend money on things. Nintendo could sell a few million units.
There are a couple caveats. Wii U "doing well" does not mean it will do better than PS4 even with a stronger lineup of games, nor does it mean it will "do well enough to consider this anything but a wasted generation for Nintendo." While this should be Wii U's best holiday season yet (and probably ever, even if Zelda comes out next year), it still won't be an especially great one in the grand scheme of things.