Its a risk...

They need to make sure this doesnt just sell to gamers.
It needs to have a stapple of normal apps, and do things people expect of a tablet, so it can sell to casuals too.

The Switch trailer featured no children, just 20-somethings while 3DS ads typically are aimed at children/parents.

That makes me believe Switch will be marketed as a more premium, high-end device and 3DS will remain their low cost kid-friendly device for a couple years until 3DS support dries up and Switch price comes down.

good point.   i look at it like this, if nintendo had been selling a "vita" as their handheld alongside wii u,
then yeah, this would be replacing both.  but 3DS was very far from vita, it's quite distinct.
sure, to some degree, some fans might just use Switch instead of 3DS, and 3DS sales might reduce somewhat.
but that doesn't mean they can't exist simultaneously.  hell, PS4 was co-sold with PS3, after all.
for all we know, nintendo is separately planning a 3DS2.  certainly it should be eligible for an update,
what with 14nm/10nm process which would also allow shrinking battery size for same performance.

Nintendo always releases revisions so i think in a couple years they will have a smaller Switch model that can fully replace the 3DS line.

But much bigger profit because now they can focus on a single system pipeline for games and marketing.

I don't think that's a good way to look at it. 50 million is good for any console it doesn't matter what kind it is or what it's replacing.

Quite honestly, the Switch doesn't seem like a very good handheld. I see no reason why they can't make a dedicated handheld to complement it, especially one that offered a full selection of mobile games.

70 million+ is probably asking for too much. It's not like there are 70+ million Wii U and 3DS owners anyway, most Wii U owners for example probably have a 3DS for example.

My frog,  always go with shipments when you make that kind of  projection ( most of the time you sell what you have shipped, in the end) , so sell-in is better for lifetime sales prediction...

 3ds+wii u are at 73m as of june 2017,  so does the 75-80m range seem to you right? It could be at 75m as of September 2017...

I think it's doable.