Make no mistake, a new MK game is always in development in some capacity and MK9 most certainly is - But since MK8DX was a massive success right out of the game, and has continued to be so for more than 2 years now initial launching plan may very well have changed. Nintendo is no stranger to holding games, and in this case it would make perfect case.
So you think they've basically paused development on Mario Kart 9 because of MK8DX's success? I know they've held back some games before, but typically only for a few months to avoid a clash with another game. This would be on another level altogether.
As far as I'm aware, EAD9 has only released ARMS and MK8DX since Mario Kart 8 released in May 2014, right? So they're definitely working on
something at the moment, and my main argument is that considering the Switch was basically a hail mary system for Nintendo (with no cheap handheld to fall back on if it bombed),
surely they wouldn't have been banking on a Wii U port of Mario Kart 8 being the success it turned out to be?
I guess we'll know for sure next year, but I'm betting on Mario Kart 9. Hopefully you're right though - I'd honestly prefer for them to be working on something else.