One successful system is better than one succesful and one failure though. It's just about about the raw number. I think that comparison is silly.
I don't really understand what you mean. What it's about, in business, is the bottom line. Revenue, profit, and growth. Nintendo, and every other corporation, will be judged by the numbers they produce and against the numbers they've produced in the past. "One successful system is better than one succesful and one failure" is an irrelevant statement unless it translates to greater overall success.
The Switch project needs to be within shouting distance of the 3DS/Wii U era in those primary criteria. Total units sold is somewhat less of a focus, as many Nintendo fans owned both a 3DS and Wii U, but it's still important that they demonstrate good market performance for the brand. Of much more importance is that the Switch moves a ton of software and has a much higher attach rate than the 3DS/Wii U. That would show that the margins make up for the drop in volume and turn each Switch sold into a high profit per device performer. This is the strategy Nintendo is banking on.