3DS sold 6 Million units last FY, it's slowing down greatly in Japan, where this time last year 3DS had sold 1 Million units for the year, this year it's ~330K. It's software sales have fallen off quite a bit too and new entries into big series are not hitting as hard.
Nintendo usually supports their handhelds for about 2 years after the successor comes to market, this is true with the DS and GBA, people are just caught up in the moment here and jumping to conclusions, but Nintendo has already moved Pokemon to Switch, there is no reason to think that they would continue on with the 3DS platform, or create a new separate platform, when Iwata said that future hardware would be like brothers in a family of systems and stated that they would chase an iOS/Android type of platform, supported via OS/Accounts and not hardware.
A Switch Mini, with all it's circuit boards unified, passive cooling and no joycons or dock would easily reduce the price below $300, they already sell a Switch in Japan for about $240 without the dock and no other cost cutting measures.