Depends on how low they can get the price of the "new 2ds xl".
And how long 3rd party keep up game support for it.

It would take a new Pokémon game and possibly a new Mario or other high profile title. 70-75 million is a given though.

Up to Nintendo. They are expecting another 6 million sales this year, assuming they meet that goal they'll be within spitting distance. At that point it's a question of whether they continue to market the thing. If they keep pushing $99 deals during Holidays I think it can coast past 80m in a few years.

It's more down to if they continue to procude and ship the thing. I expect the 3ds will be discontinued and the last unit to be shipped next year.

No, but it will get close. It could definitively reach the 75m the moment the 2DS gets to 99$ and the 2DS 49$, but Nintendo is going to go all out for the Switch, they NEED it to sell well, it will have to hold the whole company by itself.

I doubt it. But I do hope Nintendo comes out with a new dedicated handheld because I feel the Switch is a poor substitute. Don't get me wrong, I think Switch is amazing and I love mine but as a dedicated handheld? No way. The magic of those things is missing entirely for me. There's just something... not there.

How about the return of the Gameboy line? No gimmicks, just a bad ass HD screen with two analog sticks and buttons. Think of it like a mini Switch without removable sides and it's own OS and library. That would be awesome.

It will once Nintendo gives me my New 2DS, non XL.

If Nintendo ship 6 million units over the next 12 months, why on Earth would they discontinue the system in 2018? 6 million is obviously a decline on previous years, but it's still more than Wii U shipped in any single financial year. Nintendo continued to ship the GBA to Europe and Japan into 2009, even though shipments were in the hundreds of thousands rather than millions. Production might cease, but shipments don't necessarily cease too. Nintendo continued to ship Wii in 2015 despite shipment numbers dropping into the 100,000s, too. 

3DS may very well cease to be a core part of Nintendo's business, and shipments might decline substantially in 2018, but don't expect discontinuation or an end to shipments until 2019 or 2020.

in the end it can - 8 more millions this year.

If the Switch hadn't have released, yes. But since it has, no