Before further discussion, what do you exactly mean when you say "heavily MK8 based"
Hypothetically speaking, we have the next 3D Mario, Zelda U, Pikmin 4 and possibly Retro's new game for the first year. Why waste their big guns all at once?
Releasing a brand new Mario Kart game the second (or even third year) of the console's lifespan would keep the momentum going for the rest of the gen with DLC support, just like MK8.
As in, based on its engine. No need to develop a new renderer or shaders etc. No need for testing new handling or items. The core game mechanics and its technology lifted out entirely, just like Ocarina of Time -> Majora's Mask. Similar to making a stand-alone expansion pack.
It's really not complicated.
As for the bolded, if that's Nintendo's attitude then they're simply fucked. They need to hit like a meteorite at launch. Nothing else will do. Pikmin 4 will not shift consoles to the mass market at launch.