You have a right to be a little worried.  However, Nintendo has only killed off one system due to low sales (Virtual Boy) which didn't even break 1m sold before they pulled the plug.  Nintendo won't pull a Sega in the home console market by releasing too many systems/add ons back to back (CD, 32X , Saturn, Dreamcast).  They will ride this generation out till they are ready with their next system.

I believe they are merely trying to discuss vague long term strategies to calm their stock investors.  Their stock has taken a hit and they had to respond about their future goals and plans.  There is nothing wrong with planning for the future and the only reason why they discussed it vaguely was to calm all the doom, mobile, and third party calls.  Pretty much Nintendo is trying to politely tell the video game "analyst" to shut the fuck up.

I am fine with a successor in 3 years but not 2 years

Of course it shouldn't.

It's logical they'd already be thinking about the next thing. The hardware development teams aren't going to do nothing for a couple of years; when a product's done, it on to the next. It isn't any different for the other companies. Iwata only mentioned some of the things they've been brainstorming about because of his talks about long-term prospects. You can safely buy a WiiU now and be quite sure it'll be supported for at least another three, likely four years .

a successor in 3 years would also mean much less exclusives games for Wii U ( or even no more exclusives ) in 2 years to be ready for their next launch, like they did with GC

If I was going to buy a $300 console it better last me more than 4 years
If true where looking at 2 big releases per year, put it 4 years that's only 8 games!
Fuck that not worth it in my opinion

No. If you do so, you'll miss a bunch of amazing games and the next Nintendo console won't be coming for another 4 to 5 years. So guy buy a WiiU already.

When some games offer 100+ hours in gameplay then it might be worth it to others. 

We aren't stupid. We know most devs do research for next gen consoles pretty soon after the launch. But it's should be a huge red flag that we're hearing news about it less than two year after it's predecessor's launch.

Without dlc and online multiplayer how is that possible?

Not worth it in my opinion, but hey if you think 8 games is justifiable than good for you

Most Nintendo games don't need DLC to put in over 100+ hours of gameplay (Animal Crossing is a prime example).  Also, what lack of online multiplayer?  There have only been a few games that axed the multiplayer out of Wii U versions of games and those are third party games.  Mario Kart will have excellent online play and hopefully Smash on Wii U will fix the lag that was in Brawl's online play.  People still love to act like Nintendo's online offerings are ancient.  What does Live or PSN offer that Nintendo doesn't for online gameplay?  Cross game chat?  Big freaking deal 

Just going to remind you that they no longer use friend codes for each game because there are still some people that believe they still do even though they stopped doing it with the 3DS.  One more key thing... You don't have to pay to play online.