Some people are still doubtful regarding Nintendo's multiple statements about their plans to support the 3DS in the future because of what happened with the GBA line and the Wii U in the past. Nintendo also expressed support for these consoles even though the DS and Switch were already announced during their era but they were eventually dropped in the end. There's also the fact that future main Fire Emblem and Pokemon games will now only release exclusively on the Switch. Nintendo has once again abolished these doubts with their recent affirmation about the subject.
In an interview with Japanese news outlet Kyoto Shimbun, Nintendo President Tatsumi Kimishima reinstated their previously announced plans to continue 3DS support throughout 2018 and beyond.
Nintendo clarified that the Switch's current growth and success will not undermine the 3DS at all. The 3DS systems are currently selling at a rate that matches their own projections so they will not be removed from store shelves anytime soon. Kimishima also mentioned that the 3DS platform still offers unique elements separate from the Switch experience.
The latest iteration on the long-running 3DS family is the New 2DS XL which became available locally starting July last year. Nintendo has not commented on any possible "3DS successor" yet and they still don't consider the Switch to be a direct successor to the Wii U or the 3DS. While they have a lineup of 3DS games for 2018, we may have to wait until 2019 for more announcements. Only time can tell.
Source: Kyoto Shimbun