"it will take time to develop something better."
How much time do you believe they need? This is the seventh year since Mario Kart 8 first released. Are you people even counting? They should have enough time to release a new Mario Kart this year, and then still have another one cooking to be released for the next Switch by year 3 or even year 2, as the Switch still has crazy momentum and won't need replacing any time soon. Mario Kart 9 on switch is the most logical course of action to ensure that momentum keeps rolling.
They haven't released a single new DLC pack or track for the game since early 2015. I played hundreds of hours on WiiU, and even though I always plan to buy it again for Switch, part of the reason I put it off is I still have mental fatigue from the time I sunk in. Even Switch only owners will be reaching that point sooner than later.
Why would you want to blue-ball that huge player base while missing out on massive guaranteed sales, along with a big late-life boost for the Switch, just to put it off several more years for a system we know nothing about?