We know absolutely nothing about NX or what it is. Heck, it could be a Wii fit style dowacky. All I know is that this question is like asking what people think of the best football player in the year 2057. 

It will be worse as wii u if it is a home console.
If its home console + portable worse then wii u + 3ds combined.

There are alot good games already on wii u, still nobody buys the system.
This wont change.
Im sure that almost actual wii u owner would get the nx (as a home console). People get the ps4/xbox one because they want a system they can use for 6+ years. They wont switch to nintendo after 3 years. No way. They have their favourite franchises, their friends and clan on these systems. They dont want to have 2 systems (otherwhise they would already get the wii u). They already invest money in the system (more controllers, camera, maybe hololens/project morpheus).
They wont switch to nintendo.

And even wii u owner wont get the system after nintendo stopped the software support after 3 years.

So who should buy the system?

Also to all hardcore nintendo fans:

Dont expect to get all your awesome games for that system. Nintendo is in the amiibo money mode. They probl will release only amiibo related games on the new system. Aimed at casuals and kids. Have fun, im out.

I second this. All the games for Wii U are supposedly moved to the NX so that means that it will be super sucessful

We don't know nearly enough about it to guess something like that.

Well, if gamers classify it as part of this gen, then it automatically becomes last place and will be compared to Xbox One and especially PS4 sales. Automatic failure if you look at it like that.

Or it could launch the "next" gen and become the default winner until PS5 and the next Xbox show up. Automatic success.

The gamers will decide which gen it belongs to

if they release it at the same time as the competition ...very successful.

if it's a console, I'm very sure it will sell worse than WiiU at launch. (since it had a really good launch)
No clue about the rest though.

it wont. people who care about nintendo already got a wii u. and you see, thats not many.

If it is a console I think it will be either like NES or the last nail in the coffin.

If it is a handheld it will do 60m+

If it is a Tablet it will do between 10m+ yearly if it includes yearly updates

If it is a handheld/console Fusion: No idea

I have a feeling its another gamecube