The PS4 has shown that Knack is the only system seller out there, so MK8 wont have an impact at all! :P
I think MK8 and Smash Bros. will move a lot of consoles, but it wont be enough to keep the console alive for a few years. I dont think Nintendo has a bigger system seller coming out the next year than this year. Zelda and a 3D Mario are nice, but they dont have that much of an impact compared to Mario Kart.
If you're interested in Nintendo content and understand german, maybe you want to take a look at my youtube channel! :)
https://www.youtube.com/user/RealGamingExpert









