I don't think it's just Nintendo that are moving too fast; the industry in general is moving too fast. I recall Sony saying that the PS3 would have a ten year life cycle...well, if that were the case, the PS4 wouldn't be coming out until next year. But then, they're still making PS3 games, so until they stop I'd say that it's still going strong. I'd also say the same applies to Nintendo: I wouldn't consider the Wii U or the 3DS supplanted by the next generation until the games stop coming. I'm not concerned about their next project, so long as they continue to support the Wii U and 3DS for a few more years.
On the subject of Nintendo: it's Nintendo. I don't see any point in getting worried about their future plans until they happen. I mean, nobody thought that the Wii or DS was going to sell well at all...the Wii in particular. Both were hugely successful. Nintendo have (or had, considering how poorly the Wii U is doing) a way of exceeding expectations, and they make consistently high quality games...barring the odd exception here and there. On the surface, it's quite alarming that they're already planning another device, and they've made some questionable decisions of late, but...well, I think they'll surprise everyone. It's what they do. I'm not going to start panicking until I see exactly what it is they've got planned, regardless of what the signs point towards. With the New 3DS only just released, they're not going to be discarding the current generation any time soon, so what harm if they introduce the next generation next year, or the year after? It's how they stay ahead in the industry...and considering their unique position within said industry, they need that advantage.
Consoles are expensive, and they're long-term investments. I'd expect a console to last at least seven or eight years, which I don't think is unreasonable. If they want to release the next generation in the meantime, that's fine, just as long as they continue to support the previous generation for a time. As there are considerably more developers out there now working on games for these consoles I'm not too worried about the next gen arriving sooner rather than later either - there are still going to be a huge number of games on the system. That, plus it takes the next generation about two years to build up a decent library anyway...PS4 and Xbone still don't really have one in my opinion.
tl;dr I won't start moaning about generations getting shorter until games stop coming for them within about a year of the next gen coming out.