ha you're telling me basically 4k gaming is not for tomorrow, even with old gen graphics
That's a shame..
It's not impossible, but if you want to run games at 4K/60fps (which would be necessary for competitive games like Smash Bros, Mario Kart and Splatoon), you'd definitely have to tailor the console specifically to that need. This means spending more money on RAM, and less money on the SoC, which inherently means a console that's less capable at 1080p than a similarly priced device designed specifically for 1080p games.
Now for the Splatoon and MK sequel, i wouldn't be as sure as you of that. Nintendo has learned how to milk a game community now and they'll prefer to release an HD port with exclusive DLC, and even maybe a small price for existent users. That's still free money, and they don't break the community. Splatoon especially is just in the beginning of its life.
The big reason that Nintendo have only ever released one Mario Kart game per generation is that the considerable majority of people will only buy one Mario Kart game per generation. It's primarily a competitive, multiplayer game, and there doesn't tend to be big changes in the formula from one game to the next. There isn't a new story for people to pick up the newest edition, nor do they have the "roster updates" excuse that brings people to buy near-identical sports games year after year. Effectively, if Nintendo release Mario Kart 8 4K edition near launch, then people will just keep playing it until the end of the generation. It's a damn good game, and it's not going to stop being a good game when Mario Kart 9 comes out, so why upgrade?
Besides, Mario Kart 9 could be ready very close to the console's launch. Mario Kart 8 came out almost exactly two and a half years after Mario Kart 7, and if you plot two and a half years after the release of MK8, you've got a date in late November 2016, pretty much exactly when we're expecting NX to launch.