If NX fails they'll get out of the console business. Probably keep making handhelds of some sort, but for consoles they'll support other platforms (Playstation, XBox, STEAM, whatever) most likely.
That'll be 3 out of 4 "failed" consoles to some degree (GCN, Wii U, and NX) if that happens and really even the N64 could be seen as a bit of a dud considering it was routed by the Playstation and ceded Nintendo's leadership position in the market.
So the stakes are high with NX. They can't f*ck it up.