MK8 sales are heavily tied to hardware sales and Switch hardware sales will drop a lot this year. If MK9 wasn't coming then people picking up a Switch 2 may have got a copy too but as it stands. The combination of Switch 1 hardware declining and Switch 2 getting it's own Mario Kart means MK8 sales are going to start dropping pretty fast.
Despite this I think it will probably still do it anyway. MK8 sales will start dropping a lot, particularly this holiday season but I don't think by enough to stop it reaching that goal.