The portable console time has passed, they need to move to the new market (phones/tablets) otherwise they will fail.
Nintendo is still hugely successful for hand held gaming devices, or what you called the portable console.
The 3DS' was an extremely expensive handheld priced $50 below a PS3 or decent Xbox bundle, and its only real differentiating factor from the DS was a gimmicky glassless 3D. EVERY company is struggling with 3D because of lack of consumer interest and the extra cost at the check out.
Nintendo hardly needs to move to a new market, but they need to tidy up a lot of the mess they've made.
Their biggest problem is "too much kid factor" as the majority of their games on all platforms aren't appealing to adults.
The Wii may not be selling like hotcakes anymore, but its still outsold the other 2 consoles in the market. The problem with the Wii is also games. Nintendo did not make it easy for third party developers to create games that took full advantage of the Wii's motion controls (so lots of bad game play, and lots of crap titles or games that suffered from bad ports). Lots of game studios abandoned the Wii.
Then there's Nintendo who put out great games for the Wii, but very few of them. Nintendo hit a home run with innovation, but struck out with hardware limitations.
Their new console looks interesting, works with the Wii remotes, but still not many details as to what will be under the hood of that thing either.
But a dead market? Not in the slightest. No one has been able to compete against Nintendo with handhelds. Really, why would anyone buy a $250 hand held that doesn't do any more than the $99 or $149 flavors in the DS line? 3D? Yeah, not a selling point for double the price and then some.
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As for Nintendo porting games or making games for any other platform, hell will freeze over. Nintendo views Apple as their only competitor in hand held gaming. They see Apple as Apple sees Microsoft. Nintendo has an echo system to protect.
Not that I wouldn't love some super mario bros on my iphone without jailbreaking.