With the Nintendo Switch being both a, handheld and home console (hybrid); it will need to have an increase in hardware sales when factoring in the last 5-6 years of the 3DS and Wii U sales combined. A Switch Mini will probably take over the handheld console slot, but what's different is the same cartridge will be utilized on both devices (no need to purchase software twice). Less software being sold means more hardware sold is needed to offset potential losses from the previous generation. 80 million is being conservative, when factoring in 2 console's hardware sales.
If we are going to judge Nintendo's success on there being more interest, customers, and profit than time's past, then optics is very important. Noone is going to want to purchace a device that game developers and consumers are unsure of. Sure Nintendo is going to get the console launch sales boost along with the oh wow I've got to have it because it's new and it's hard to get. Hype will also lead to sells but that's not what is needed to sustain the Switches lifespan.
Utimately it will be Nintendo's 1st party games that will bring in the largest chunk of the previous fanbase and Nintendo's ability to bring in 3rd party AAA titles. Game droughts will only hurt (obvious). Proper marketing and consumer targeting will create a winning environment.
Anything less than 80 million in hardware sales in 5 years will show a diminishing interest in the "Nintendo Brand" and unfortunately create another hurdle that will need to be cleared. They are off to an excellent start, 4 years and 10 months to go.
(Full discloser: I will buy the Switch for Xenoblade 2 alone, even if it's the only game I will purchase.)
...to avoid getting banned for inactivity, I may have to resort to comments that are of a lower overall quality and or beneath my moral standards.