As for the Switch: I'm agreeing with what others have implied, which is that the Switch has no reason not to be successful... it's just that it's a matter of time. Comparisons have been made to the 3DS, which also started slow, but eventually went up in sales due to increasing support. I know it's kind of a simplistic/cop-outy answer, but it really just depends on Nintendo. People want Metroid and old IPs revived, and it needs 3rd party support; if it gets those properly this time around, I think it should do just fine. And especially with the way Xbox/PS4 stuff has been going, Nintendo really needs to work on differentiating itself and putting out console exclusives that are worth picking up a Switch for. Obviously, from a tech perspective, it won't be competing with an Xbox/PS4, so it has to compete in other ways.
Fun note, Ben Heck did a breakdown of the switch, and found that the RAM (I think? It was some component) was made to be easily replaceable; He suspects we'll see a Switch with upgraded hardware in the near future. Others have noted that Nintendo had patents or copyrights or something like that mentioning VR-esque tech. So it's very possible that the Switch could also act as Nintendo's VR-esque machine, which (at it's price point) could help to sell it and make it very competitive, with the proper game support.
As for the 3DS: business.financialpost.com says here that "Nintendo president Tatsumi Kimishima went on record last fall saying that while the company's upcoming hybrid console, dubbed Switch, will presumably succeed its home console, the Wii U, it will not replace its portable game machine, Nintendo 3DS.Jan 25, 2017." That's from Google, and just one source, but it is a statement from the President, himself. I think they will continue to have a completely independent, portable platform. The *new* 3DS hasn't released all that many titles geared toward it specifically, but as Indenton said, playing games like Pokemon Sun on an old 3DS XL has resulted in really noticeable issues. Sony hasn't spoken a word about a follow-up to the Vita, and they were basically the only competitor this generation for mobile stuff (aside from tablets/phones obviously). Until this happens, I think Nintendo may be comfortable with releasing games for the *new* 3DS, as we are seeing with Fire Emblem titles.
Now, I guess it always *could* happen that the Switch becomes so successful that they decide to focus on just that, but I find it unlikely. Despite it's portability, I'm not exactly sure that it can replace the portability a 3DS has in an urban environment like Tokyo, etc. They also still own the GameBoy name; could be the next installment is a new GameBoy after the 3DS. Who knows?
TL;DR - *New* 3DS doesn't have any competitors, and graphical power (not to be confused with presentation) has never been Nintendo's strong point or concern, for the most part. Fire Emblem indicates it still has some life left it officially, and unless Nintendo feels compelled to replace it, there's not really any reason to for now. I don't think the Switch will effect that much, if at all.
Someone mentioned 2018... that sounds pretty good to me. I guess it's just a question of where will they go from here? Who knows, maybe it will be a sort of Switch companion; a much smaller version that uses the same controllers, perhaps? Imagine a GBA SP, but the bottom are the two detachable controllers... could be interesting haha.
Though now I'm starting to seriously wonder what we might see integrated, either in philosophy or shared components, from the Switch in a new portable console. I could definitely see cross-platform capabilities/play.
