There are a bunch of people who have held out on getting a Wii U until now, and 3D World will move a lot of units for sure. I'm a big Nintendo guy, and I haven't gotten a Wii U yet. I should be getting one later this year, however, and 3D World will be the first game I get. The announcement of Mario Kart 8 is the main reason I'm going with a Wii U, but 3D World will definitely end up in the top 5 Wii U titles of all time when the system is all said and done sometime in the future.
I definitely think a lot of people will come on board the Wii U bandwagon for this game, but if for some reason that doesn't happen, I think it's fair to worry a little. The real worry will come if MK8 and Smash Bros. Wii U don't move units, but this is definitely one of Nintendo's huge titles.