As long as Nintendo stays with hybrid consoles, they are guaranteed success. Of course, I don't put it past Nintendo to stop doing hybrid consoles. That is the one way they could screw things up for themselvs, and I do think they are likely to keep making hybrid systems with each generation. But I've also seen Nintendo turn away from the guaranteed successful route enough times to know that they might do it again.
So, assuming they stick with hybrid consoles, then I don't think they will ever sell less than 85m no matter how they screw up anything else. Nintendo has never been defeated in the handheld market. Sony gave a real good try at it and so did Sega and a lot of other companies. Nintendo just can't be beated in the handheld market. The worst screwup they had woud be Generation 8, and 3DS + Wii U was still around 88m. That is why I say 85m is the absolute floor on a hybrid. Even if they do everything wrong, like they did for Generation 8, they are still going to sell at least 85m.