In Japan I could see it go ten years. I do wonder how Nintendo's successor compares power wise to the Vita.

And the horseless carriage was a bad idea? No idea how an iPhone is relevant. Music does not require any kind of interaction other than listening which can be done while driving, while working, while playing sports, working out, sex, literally anything. It's hands free.

Carrying around a vita phone - at best, form factor would be that of the Z3 meaning the tech behind it couldn't play anything better than what we already see on mobile platforms.

At worst, it will be the size of a fridge and be an absolute abomination of an eye sore. Sorry but it doesn't make sense. Let alone the investment making an OS for a cell phone that's even remotely as advanced as android or iOS would cost. There's quite a laundry list of con's to the few benefits.

Until the Japanese devs stop mistaking it for mobile.

I'm guessing it will be discontinued is about 2-3years...

What makes you think the NX will be more successfull with third party games? 3DS has 5 to 1 of the installbase but yet third parties are prefering the vita. I think its a sign that handheld gaming is dying, you maybe see nintendo games selling well but third parties are going away in the first place.

Look how much the investment of vita costed, they could easilly make a phone with similiar specs and the same OS, have you ever had a vita tv in your hand? People espacially japanese would buy it and sony wouldnt make any loses as they could charge you 500$ or more.

There are a lot of benefits, like owning the ecosystem in fact sony allready has an eco system with the vita so there goes the massive expenses youre talking about too.

up to 2017 in the west
up to 2019 for japan

Monster Hunter? Dragon Quest? Bravely Default? Shin Megami Tensei? Kingdom Heats 3D? 

Sure, niche third party developers are taking advantage of the Vita's market, but that doesn't mean the 3DS hasn't had decent third party support, especially from top-tier developers like Capcom, Square-Enix, and even Atlus supports the 3DS with a main series SMT, and various ports of side-series. 

Well if the third party is going away, then the Vita is dead and if the third party is staying, then they will be staying on the NX because it will be more powerful than the Vita since that is how it has always been in terms of hardware. The issue is that the engines they used on the vita won't really work on the 3ds due to the performance gap and I am sure that many third parties thought that the Vita would be much more successful than it is so they probably built the engines for the Vita but redoing the engines for the 3ds would probably take too much effort.

So if the NX is more powerful than the vita, then the third parties shouldn't have much of an issue reworking their engines to adapt to the NX so thats what I think will happen anyway. And its not like the 3ds doesn't have third party games because it has quite a few that are exclusive to the 3ds where as I would argue that many third party games on the vita are either avaliable for other platforms or the other platforms give the better experience (such as Cod vita vs Cod console). Not to say that I am saying the Vita doesn't have any original third party games but in terms of third party exclusives, I don't think the 3ds is that far behind

Seeing how Sony`s consoles go, it`ll be around for a long, long time. In Japan it`ll have games be developed for it likely 5 plus years from now. They keep their stuff floating.