Each one of those is from a console that launched later in its respective gen, with the only exception is the DS, which really only launched so early because they didn't want Sony to take over the handheld market like they did with the PS1. So, it's not really surprising they would have a shorter timespan before a successor launched. It's also important to note that the Master System and Xbox were Sega and MS's first real attempts at console gaming. We're talking about a console that launched a year earlier than its competitors and after a highly successful system. The Wii U's situation would more closely resemble the Saturn's. Launching ahead of its competition and after the highly successful Genesis.
The real problem Nintendo faces is that it's home consoles have been declining in demand among core gamers from the beginning. NES 62M > SNES 49M > N64 33M > GC 22M. Wii U may continue this trend and sell ~20M, less if they jump ship early. The only exception to this trend was the Wii. But that had the added benefit of taking advantage of the growing casual market, something that has, for the most part, either gone to cell phones or just stopped gaming, and is not coming back. And its success definitely did not stop the success of the systems more focused on core gaming.
If Nintendo was smart, it would just abandon home console gaming and stick to handheld gaming, focusing all of their major exclusives on 3DS2. Or if it wanted to put out some more ambitious titles, occassionally dabble in 3rd party publishing.
Nintendo's situation is not like Sega, in actuality Sega's situation really was nothing like anything in the industry.
They launched the Sega CD in 1992 for $300 ... which was double the price of the Genesis at the time would be $474 in today's dollars (think about that for a second). After 2 years of wishy washy support for the Sega CD, they then launched the Sega 32X, which they supported for oh ... three months.
They then "surprise" launched the Sega Saturn out of nowhere in spring 1995, just seven months after the Sega 32X.
And they basically gave up on the Saturn by 1998 (they tried through fall 1997 with that three free games Saturn promotion and basically threw in the towel after).
So no. People who play the "Sega card" need to actualy do their home work.
And really, even after *all* that (lol), they actually had a very solid Dreamcast launch, the Dreamcast fell apart once it was clear Sega couldn't financially support it.
The NX platform could basically addresses your later point ... by having a platform that shares games between portable and home console, they basically are "going handheld" but still can enjoy the perks of having a dedicated console (ie: sales of controllers and accessories).
While Sega was foolish with the Genesis add-ons, doesn't really change my point. The Wii U also had a solid launch. But, just like with the Dreamcast, things change when a competitor has a more wanted product. And no, a NX platform would just mean there is even less reason to own the home console, since any of its games can just be played on the handheld. This would cause unecessary costs of producing an unwanted home console and its various peripherals.
Nintendo needs to focus on handheld gaming and try to stop the decline there. Otherwise they may start losing their real cash cow. Nintendos best bet would be to make a decently powerful handheld. Throw a mini-HDMI port on it and allow games to be played with Bluetooth controllers for TV gaming. This would also mean no more droughts and plenty of 3rd party support.
I think they are kind of doing what you're suggesting in that last paragraph, but just without the HDMI port. Just like Apple doesn't give you an iPad for free just because you own an iPhone, Nintendo will still expect to to buy a seperate console to play those same games on the TV.
I mean heck even with the Vita ... you still have to buy a PS Vita to buy the games on TV.
Companies generally tend not to give away added functionality if they know they can get away with charging seperately for it.
But in terms of shared library and no more/less droughts ... yeah I think that's happening.