If I bought a Wii U at launch then I would have considered it a mistake, but at the point which I got one it was pretty clear where it was headed and I knew what I was getting into. The system actually had a pretty successful launch month and a selection of third-party games in series that the Wii didn't get, so the future showed some promise. But that's why I don't buy systems at launch, and wait until the games I want are actually out or close to release. I knew it was just Nintendo titles and there were enough to make it worthwhile for me.
That said, the system is really disappointing as a followup of the Wii. Despite its reputation the Wii actually had a good selection of fun and quirky third-party titles, and the Wii U isn't even getting those kinds of games. And Nintendo's own output doesn't feel as strong as the Wii either; they've released less games and I don't feel they are as good as what was on Wii. With Nintendo being the sole supporter now, they should release more games than its predecessor, not less.
I hope Nintendo have learned their lessons but at the same time, some of their issues have been evident for a while and should have already been fixed with Wii U. There are still some signs that Nintendo are improving, but I'm not convinced they even fully understand all the reasons the Wii U is a failure. Even if I don't regret getting a Wii U, I don't think I'll be on board for the next system if it's the same thing.
I'm not sure how much this relates to Iwata's comment as much as the discussion in the comments, but, uh, yeah, I don't think they really have a choice but to stick with Wii U for now. They can't afford to rush into a new system to replace it, they need to take their time and make sure they not only make a good next system but also have quality games available at and near launch. And so even if they do realise the mistakes with Wii U, they can't talk about them yet because they're still trying to sell the thing and make some money on it as they plan out what they will do next. They're in a tough spot, but I feel this is the best course of action.