It isn't just Shovel Knight. A lot of indie games are actually doing very well on Wii U, and the fact of the matter is, Wii U is a thriving platform for indie companies. A larger majority of the people who do own Wii U's right now, are also the type of people who want and would download digital games, as well as buying retail. That is also a big part of why seemingly every week, you hear news of some new kickstarter or funded indie project, announced as coming to Wii U eShop. The 3DS eShop is also thriving incredibly well. But I think now you can finally say that the Wii U eShop is finally starting to really rev it's engines, and catch up to 3DS.
Just mere months ago the amount of games releases on eShop in a given month was still relatively small. But now it's exploding, because more and more of these games are getting finished and released finally, such as Shovel Knight. As a Wii U and PC owner this gen, I can say that I'm thoroughly satisfied with having Nintendo first party titles and awesome indie games. I'm not really sure that I need much more than Wii U and Steam, to be honest.
P.S. Shovel Knight is awesome playing off-tv on the GamePad.
