I don't think the 3DS was a failure at all. More like a moderate success in comparison to the DS, but not a failure.
I'm pretty confident that a lot of 3DS and Vita devs will come to the system. It could be a great place for more medium budget games (though I'd bet it'll get a few AAA games as well). If they really do have DQXI and are able hold on to Monster Hunter, Japan will already be interested. Not to mention Animal Crossing, Pokemon, etc.
The potential for a great system is there. It will at very least do far better than the Wii U I believe.
Exactly, the 3DS is going to sell 70-80 million units, and that's very impressive for a device that competes with smart devices. Some people will try to skew the perspective that it didn't sell nearly as well as the DS, but the DS is the highest selling piece of gaming hardware ever. Its like saying the PS4 will be a failure because it didn't sell as well as the PS2 did.
3DS and Vita devs will most certainly jump on board with Switch, its going to be huge in Japan. Nintendo alone will get the Switch out of the gate with Splatoon and Mario Kart, and probably a Monster Hunter from Capcom, all but making the Switch a huge success in Japan. The rest of the world is harder to judge, but we certainly did see high profile games from Nintendo spike Wii U sales. They surged with Mario 3D World, and then again with Smash and Mario Kart 8. If Nintendo can consistently roll out high profile first party games, they will drive the userbase, and third parties will certainly be there if the userbase is growing at a fast rate.