Many people have asked for a similar model for sports games and while I would like it, I doubt it will happen. I'm not sure if it would bring in the same amount of revenue as releasing a new game and supporting it with well done DLC throughout the generation.

I'd argue the opposite would happen. I think they could stand to make even more money. It's a model that made The Sims a mega hit and one that works for MMOs as well. People like to rag on Street Fighter IV, but it does a lot of the same things as well.

Theres a lot of games that should take this route in my opinion, like the new Gran Turismo or Rock Band and what not. We just need 1 game, maybe even free with a special retail pack. Then you can make DLC for the rest of the generation.
But yeah the next MK should definitely be a platform, Splatoon looks like a good contender too. Dont really care for SSB that much

That implies that the next Mario Kart game will be the last ground-up Mario Kart game. I personally dont like that idea if they're gonna stick to the same graphics engine whatever the NX is going to have, "indefinitely". Building a game around a let's say a 1 teraflop console and sticking to this same exact engine forever just for the sake of keeping the same exact game going will lead to limitations, there will be no more new engines for the game, it'll be the same one, with minor improvements with patches. 10Teraflops + will be able to do much more complex scenery, higher polygon models, better resolution, and higher quality lighting than whatever their next console will do, these things won't just " magically" pop in with patches. Newer engines also lead to better physics, advanced effects, high quality visuals, these things will keep improving with advancements in graphics technology. People stated things about Wii, defending the low power of Wii and how the graphics looked " good enough" for them, and then they saw Mario Kart 8, telling everyone clearly, that things could look better.

Although Wii U is inferior to the competition in power, Mario Kart 8's artstyle and graphics engine showed the clear difference a generational leap in power can bring, and there are countless improvements that can be made with stronger hardware. Yes, even Mario Kart has room for lots of improvements in graphics, don't let the simple shapes and cartoony visuals fool you into thinking there's not much left that could be done. I say, just keep with the same route they're doing now, still release one Mario Kart, one Smash Bros. per console, but extend their lives with DLC. Now, if the NX platform also leads to the successor of Wii U's successor, then release Mario Kart 10 on it, taking advantage of better tech, not simply continuing Mario Kart Forever with higher resolution. I'd like to continue seeing surprises with their premieres of these games, with the generational changes a typical generational power leap comes with.

I certainly hope you don't think Nintendo will be "satisfied" with the power of their next platform enough to never create Mario Kart from scratch again. 

As you already said, Mario Kart 8 and Smash are approaching the direction. It doesn't seem like they will stop getting new content either. I personally am fine with the way it is now.