It is also a misconception that the Wii U version would even exist without the 3DS version
the 3DS version and the sales it will bring helps the Wii U version even have a life to begin with.
So, you're telling me, that out of all the people who are going to buy Wii U now, a tiny fraction would not buy Smash Wii U exclusive just because it wasn't on 3DS.
You know what...? I could sell all that salt, buy out Nintendo and make the Wii U Smash anyway.
Addressing the OP:
If Smash 4 was Wii U exclusive, the game and console sales would certainly be smaller that what they're going to be now. Actually, that doesn't matter to me.
What matters is we could have a BETTER game. Were Smash 4 console exclusive the developers wouldn't have had to make those odd decisions of cutting Ice Climbers on the Wii U version due to the 3DS.
If I had to choose between console diversity and more sales and better gameplay, not for a second would I consider the former option favourably, especially when Nintendo is already predicting a profit over 420 million dollars (370m USD)with Smash 3DS. I don't do commerce. but I have serious doubts as to the abscence of Smash 3DS setting Nintendo back hundreds of millions of dollars compared to now.
At any rate, all this salt mining can be stopped if the developers simply allowed the Wii U version to progress to its own limits, rather than the ones set on the 3DS.