Mario kart 8 has 55% attach rate, each quarter is rising by 1%, instead of falling! The more people who buy a wii u, the more Mario kart 8 sells! It's at 5.87m as of September 30th, only 1 year and 4 monts after it's release!
Wii u seems that will sell 15m at minimum, likely 16-16.5m( Nintendo expects it to be at 13m as of March 31th 2016, ergo it can surely have 2m sales onwards, that's not too big to be unfeasible, gamecube sold 4m onwards with Revolution (wii) revelead in gamecube's 4th year at E3 and with announced release date: after 1year and a half), so Mario kart 8 should be 8m at least...
With some luck ( price drops and maybe no backwards compatability on its successor, as a result securing decent legs) wii u may reach 17-18m at maximum... Then certaintly Mario kart 8 would outsell Super Mario kart... But the successor backwards compatabilty at least fosters the games legs! Mario 8 is a diamond, so people without wii u but with its successor would definitely like to not miss such a great game! Lastly Mario kart wii still pushing big sales, inspite of wii being dead!
Actually, even with wii u being at 15m, Mario kart 8 could outsell super Mario kart, with 60% attach rate, just 5% more than it has now! Keep in your mind people that as I aforesaid Mario kart 8 each quarter is increasing its attach rate!
Make a bolder prediction than this...Mario kart 8 outselling Mario kart 64 9.87m!:P