It will sell more than Wii U, it could do 30-50M imo.
30-50M is not a bad numbers, but 50M+ is a success imo.

Nintendo has been going downhill for years, and the Gamepad gimmick, although i love it, isnt what fans want. if the Nintendo NX isnt a success, it could spell out doom for Nintendo.

"Please Understand that there is a drought on Wii U because reasons"

-Iwata in 2018

I think:

under 20 million: Doomed

20-25 million: very bad

25-30 million bad

30-35 million: sufficient. ( not bad, not good )

35-40 million: great

40-50 million: good

50-70 million: very good

70-100 million: beasting

100-150 million: insanity

over 150 million: god (DS, PS2)

No. Nintendo is launching a console midway trough the generation to try and catch up yet again, theyve done that way too many times already and it never works, they should just invest on the 3DS give some WiiU suport and hope for this gen to be a short one so they could join the next with a competitive console.

Depends on the execution.

It really depends on whos in charge. Iwata and Reggie basically have no clue whats going on and i doubt they'll be smart enough to listen to what the fans want

So i think we can all agree on that NX will do worse than Gamecube but better than WiiU saleswise.

I think the NX will be launched in 2016 as a portable system. And in 2017 as a home console. Both will run the same games, have the same features and online services (and of course an unified eshop). The systems will connect basically as the Wii U connects with its gamepad now. No backward compatibility, Nintendo's going to make a lot of portings from the Wii U/ 3DS, I guess.
Anyway, while I have some doubts for the sales of the NX as a Home console, I think it's safe to assume thath the portable version of the NX will do relatively fine (think about the usual killer applications Nintendo has for its portable systems).
So if we consider the sales of both versions of the NX combined, I guess it will do quite fine.

Source?

Virtual Boy sold 770k lifetime. 

Wii U has already sold more than 12 times that.