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. 

Considering they can produce more software and sell it to a bigger userbase, I don't think they need to reach that amount.But yeah, they need to hit at least 50m to be competitive, less than that would really endanger their hardware division.

That's why they won't announce the death of the 3DS yet and surely try to let the 2 consoles coexist for a while. They need a safety net.

If the Switch flops, they kill 2 markets with one stone. It would be really bad for the company. Keeping the 3DS around can allow them to move back to it and make a real successor if the Switch doesn't sell well.

I don't see it replacing 3DS. It is going to cost quite a bit more.
No reason why they can't have 2 mobile offerings, one low-end, one high-end hybrid.
Note they've already signaled end of life for Wii U. They haven't for 3DS.

Switch could sell 200 million units and bankrupt Nintendo in the process. It's not all about units sold.

Does that mean you believe sony needs to sell about 160Million for the PS4? Because that is the logic being presented here. Because I don't think that total matters as much as the profit. (See Sony with PS4 Profits vs PS2 Profits) 

Good point.  Makes sense to keep the 3DS relevant in 2017 so if this flops they'll try their hand with a 4DS or whatever name they come up with. 

Not necessarily.. it's not always about the quantity, but the profit

I predict 3DS at 70 million lifetime and Wii U at 14 million, so 84 million combined.

I want to be optimist, and says 90 million for Switch

Anything higer than 70 million is a big success anyway

The 3DS is going to be their safety net for a bit longer because of this, there are quite a few 3DS games in the pipeline through 2017 and probably into 2018