The Nintendo Switch is rolling along nicely, but the Nintendo 3DS is still hanging in there. Nintendo's 3D handheld system is getting robust support from Nintendo through the rest of 2018, and that support will supposedly continue until people buy the Switch on a "one person per system" basis. In other words: For a long time yet.
In a recently-published investor Q&A, current Nintendo President Tatsumi Kimishima and up-and-coming President Shuntaro Furukawa talked about the company's current fortunes and its future. When the pair was asked if the 3DS can "coexist" with the Switch, Kimishima said "yes," as he doesn't anticipate parents will buy the costlier Switch for each child in a household the way they generally do with the Nintendo 3DS.
It's easy to see where Kimishima is coming from. At this point in time, the Nintendo 2DS / 3DS is a budget line. A vanilla 2DS bundled with Mario Kart 7 costs less than $80 USD. The Nintendo Switch, by contrast, is $299 USD without a game.
Nintendo already has plans for the Nintendo 3DS through 2018 and into 2019. Mainline Pokémon games are making the jump to the Nintendo Switch, but we might see more secondary 3DS games in the vein of Detective Pikachu (which is a lot of fun!) help keep interest in the handheld stoked to a low but steady burn. And thanks to Atlus, the 3DS is still dishing out some great JRPGs. Keep on chugging, little guy.