In all of their PR & advertisements, Nintendo have been advertising the Switch as a home console. It was never made to replace the 3DS & it's not even a fair comparison since it only has one screen.
Why should they be phasing out the 3DS?
The Switch is, and will continue to be, defined as a home console. While the 3DS is a handheld.
They're two different things & serve two different purposes. They can live together. Plus, Nintendo has stated that they will be living together.
It's important to note here that it isn't going to be a 3DS title, it's going to be a NEW 3DS title. This means it can probably be ported a lot easier. It'll also remove the stuffy performance that you see from the original 3DS a la Hyrule Warriors Legends.