Not really. Nintendo said several years ago - when discussing Wii U - that their hardware needs to shift 5-10 million units a year to be viable. With an install base of 66 million, 3DS is still shifting over 5 million a year, and could do for the next two years. And your assertion no-one is buying Kart 7 is easily proved false, because another 2 million people bought it in the last twelve months.
Personally I think New 2DS XL is there to test how much of a low income market there is for 3DS systems in the West. If Nintendo can find that market, they could keep shipping a couple of million units a year globally after 2019. Let's not forget GBA sold strongly in the US across 2005-2007 even with DS on the market, and whereas 3DS has sold a little over 40 million in the West, DS sold 120 million. Nintendo will want to find out exactly how much of that low income GBA/DS market is remaining. We're absolutely not going to see 3DS sky-rocket, but it could stay on Western markets for 2-4 years yet.
EDIT: I'll also point out that phasing out the standard 2DS/3DS models will make even more sense if we start to see more Switch/New 3DS cross-platform games. Part of me wonders if Pokemon Sun could be one such game.