Of course they will. MK8D doesn't count, it's a quick low-effort port with very little changes. Besides, Mario Kart sells and they know it. They're not going to pass up that chance. MK8D didn't take much time or resources to port, meaning their team has plenty of time to make the next entry in the series, and by the time that's done, sales of MK8D will have dried up and people will be ready for the next Mario Kart. Them porting MK8 just means we will have longer to wait before the next entry, we might not see it until 2020.