So as of now this generation Nintendo has sold around 70m units. Between the Wii U and 3DS. That will end up around 75-80m by the time they stop selling.
IF (key word) the Switch is a hybrid replacement for both consoles, Nintendo need thr switch to sell about 80m or they face another decline.
If it only sells 30-50m that's a pretty big decline. It has its work cut out. Now if the 3DS gets a successor that's separate to this than you can add both of them together and hitting that 80m target will be a lot easier.