I don't think a Galaxy 3 would've helped any more than 3D World would've at launch. Both would be seen as more-of-same refinements rather than proof of concept system sellers. It was exactly the same issue NSMBU faced, only that game also carried a more worn out series brand on top of everything.
The Mario game Wii U needed to launch with was really Mario Maker. Something that felt legitimately new and yet still comfortably familiar. And also something that fundamentally enforces the strengths of the platform's interface.
I mean it being at launch in addition to NSMBU - basically just having a stronger launch in general. If it could have launched w/ mario maker and splatoon in the spring, Wii U would have been a much different system than it is now, but hindsight it 20/20. Just really have to hope nintendo learned from wii u is all we can take away from any of this.