Nintendo of America president Reggie Fils-Amie, in an interview with Wired, says,

“At its heart, [Switch is] a home console that you can take with you on the go.”

That clearly tells us that Nintendo considers Switch is a home console first and not a replacement for 3DS. Replying to the question about the impact of Switch’s March launch will have on 3DS, Fils-Amie said,

“3DS has a long life in front of it,”

“We’ve already announced games that will be launching in the first couple quarters of this year. There are a number of big games coming. And in our view, the Nintendo 3DS and the Nintendo Switch are going to live side-by-side. You’re going to be meeting different price points, you’re going to be meeting different types of consumers, you’re going to have the newest, freshest content available on Nintendo Switch, you’ve got a thousand-game library available on Nintendo 3DS, plus some key new ones coming. They’re going to co-exist just fine. We’ve done this before, managing two different systems.

“I think there’s a sense that Nintendo Switch is a portable device. It is portable. But at its heart, it’s a home console that you can take with you on the go.”

So Switch’s launch won’t affect 3DS, it’s just that Switch is a home console that you can take it out with you too, as per Nintendo.