I believe Mario Kart 8 will outsell Mario Kart 7 in lifetime sales, and I have some evidence backing me up http://www.gamespot.com/forums/system-wars-314159282/mario-kart-8-outpacing-mario-kart-7-in-the-us-31350921/
1. Mario Kart 7 came out during the holiday
Mario Kart 7 launched in December, a very frutiful time for game companies to get their products selling. Mario Kart 8 came out the end of May. Many gamers may be holding back to the holiday or price cuts to get the game still, so sales will probably be high for years to come.
2. History of Mario Kart sales
Handheld sales of Mario Kart have always lagged behind console versions, despite hardware being higher in sales. Super Circuit is dead last (next to Mario Kart 8's one week of sales). Even though the DS version is 2nd, the Wii version still nearly sold 12 million more units thanks to bundling it.
3. Better rating
Mario Kart DS got a much higher rating than Mario Kart Wii on gamerankings.com, and the Wii version still sold more. Mario Kart 8 on the other hand has higher ratings than Mario Kart 7. It was more hyped and got more critical acclaim.
An incredible incentive to buy Mario Kart 8 now is the bundle going on. If you already have a Wii U you could still receive a free game with it. Also, Nintendo plans on keeping commercials of Mario Kart 8 running until the end of 2014. If they hold up this promise, it could really help, especially this holiday.
Nintendo handhelds always have many diverse games that offer great multiplayer experiences. Pokemon, Mario Kart, heck even Kirby have multiplayer fun for everybody. Granted Smash Bros has good multiplayer on consoles, but Mario Kart still remains high as the most enjoyable. 3D World had a good multiplayer system and so did New Super Mario Bros, but if you screwed up on there you were sure to get yelled at. Mario Kart you are on your own. If you screw up nobody else is getting hurt by it. Also the online is better than other series that have online integrated into their games. It works (for the most part) and is simple to understand.
6. Luigi's Death Stare
Believe it or not, "Luigi's Death Stare" has to be selling at least a few Wii U's around the world. This ties in with marketing, but I wanted to bring it up in it's own category. Not only has it been spreading around the internet like an untamed virus, but it has even appeared on Fox news! Nintendo is getting so much free marketing with people doing this. Even if 1% of 20 million people watching Fox bought a copy of the game that would still be 200k units sold.
Though many popular franchises have seem to hit a brick wall with Wii U, Mario Kart seems to be the last little light of hope for Wii U. If Nintendo does this with other huge games like Smash Bros, Zelda U, and invests in new IP's, Wii U should have no problem becoming successful.
XBOX ONE/Wii U/3DS/PC
RIP Iwata 12/6/1959-7/11/2015
Thanks for all the great memories!