If MK9 was in development and Nintendo decided to can it because of MK8 sales, I'd be pissed. That's no reason to cancel development, let the team finish it. And, no, they're not going to sit on a finished game for three years, because that would mean that they wouldn't have anything for this generation.
I'm not talking about cancelling, but rather putting it to the side so they explore additional ideas for other IPs or even potential improvements.
In any case, it's a weird situation since we're used to the MK team working on two simultaneous platforms, so a game each 3 years was always a given due to each platform requiring its own MK.