I know I've seen a great analysis of why the Wii is a step back for Nintendo, basically boiling it down to it being gimicky for the sake of being gimicky, while also trying to appeal to hardcore gamers, with a big clunky control with lots of buttons, where the Wii with its intuitive easy to pick up controls for casuals and people who don't game much gave it a much broader appeal, it was the family entertainment system for everyone, and the Wii U, it's just an Xbox/Playstation hardcore wannabe, without the publisher support of its rivals. It's no longer got mass market appeal, it's not appealing to the family and casual market, and the market it's trying to appeal to prefer its competitors. That and the branding problems where a lot of Nintendo's former casual market still think it's just a tablet peripheral for the Wii, and alienating 3rd party developers, and you got a recepie for bad sales.
I want to say it's MrBtongue or Errant Signal that made the analysis, but can't find it. It's someone similar though.
It's not going to sink Nintendo, they've got too much money, similarly to Microsoft, it's going to take a few more problems than that to bankrupt the company, but if they alienate their market, and have one or two more failed consoles afterwards, they might just end up having to cut their losses and withdraw from the hardware business, like Sega before them, or at least become a handheld-only company, abandoning the home console market.