I doubt people would stop buying the game because it was on Wii U.
Yes, some could completely reject that while others would take their time, but sales wouldn`t be that different.

It would even sell more.(on wii)-wii u is not a good/fair example as it is not the same gen as ps360

GTA may be the wrong game for the current userbase of the Wii(u) but an exclusive GTA would have had a completely different impact on Wii U demographics.

1) with a gta-exclusivity the wii(u) userbase would be a lot different than it is now=high wii(u) gta sales
2)nintendos userbase is very loyal to Nintendoexclusive games.
Having a 25% bigger userbase(100mio) than ps360(80mio) combined with nintendo-exclusiv-game-fan-loyalty the sales number should outperform the numbers of 360 or ps3(just like more than a dozen nintendo excklusiv titles have sold far more units than the best performing ps360 games(cod,halo,gt,gta)

100% sure...

My guess is that we can never know for sure

This!

However there are people that have the game now that would've never purchased the game...

Nop.

Because if that happened to any other company, nobody bats an eye (Exapmle: Dead Rising 3).

But if a game comes to a Nintendo console as a exclusive (or non-exclusive, but that's different), but wasn't exclusive before, everybody loses their mind and say "F*@!k you, Nintendo, now I have to buy a Wii U if I want to play sknvswfw game" (Example: Bayonetta 2).

No, because it took a huge install base of PS360 users to make GTAV sell how it did. Wii U is currently nowhere near that number.

I love Nintendo but no...
a) People would cry about it
b) People would just hate Nintendo for it and boycott it
c) Not enough install base

im having a hard time enjoying GTA5, it isnt a game for me but im sure if it was a wii u exclusive 80% of non nintendo gamers would buy the wii u. GTA is biggest game among the ''hardcore gamers'' and the hype built for 5 years made that game insane

I am holding out for a wii u version come on nintendo for once just bend over and suck their dicks