If not a full decade, it's going to be close. No way they release one next year. Earliest would be 2021, if we get new Switch hardware with a hardware update. I could see them keeping quiet about it this far if that's their plan. We'd likely here about it at the 2021 E3 Direct if that's the case, for a holiday release. That would still make 7 years between mainline Mario Kart releases (aka not arcade or phone games), the longest stretch ever. If there is no hardware update to justify a new entry to take advantage of it, then we likely won't see Mario Kart 9 until the Switch's successor, which will probably come in 2024, a full decade after 8. That could end up being pretty epic, as Nintendo will probably get VR figured out for the Switch successor, and fully implement it into MK9.