Not every Nintendo fan has two devices (just look instal base of Wii U and 3DS), maybe unified platform will has very good integration and people will benefit for having two devices, also unified platform means that handheld and home console will be more attractive, so such a platform could easily attract new customers.

You mean like how Windows 10 has seen millions of people buying WIndows Phones.....

Not exactly same thing, one biggest problems for Nintendo this gen is support of two completely different platforms, they cant anymore effectively support two different platforms, but with unifeld platform handheld and home console can have much better support than this gen. Platform with better support = more attractive platform.

That's not the same thing at all. Wii U would be a more attractive purchase if it had Pokemon, Animal Crossing, Luigi's Mansion, Kid Icarus, Fire Emblem, Monster Hunter, etc. just like 3DS would be a more attractive purchase if it had Splatoon, Mario Maker, Xenoblade X, Pikmin, Bayonetta, Nintendo Land, etc.

The point is if you can get all those on the one platform why would Nintendo fans buy both?  I'd say about 10m Wii U owners by the end of the gen will also own 3DS. Why would they spend a fortune buying both.  If they put all games on both straight away you're losing sales.  On top of that people are moving away from dedicated handhelds to smartphones so there will be a lot more lost sales.  

Bottom line is they won't succeed without 3Rd party support.  Because Nintendo on their own will not grow gen over gen.  They will decrease and their profits from consoles won't increase. 

First you assuming that almost every Wii U owner has 3DS also, thats not true because handheld and console gaming are not same, they are very different, so even with same games (but definitely some will be some exclusives) some people will still buy both devices, especially if home console and handheld have heavy integration. Also you talking only about Nintendo fans, but what about people that are not Nintendo fans but with unified platform they consider Nintendo handheld or home console much more attractive than before because they have much higher number of games on single device.

Yes, for bigger mass appealing NX needs 3rd party support.

Nintendo shut down their game development operations in Redmond and sold a massive parcel of commercially graded real estate about 1/3 the size of Disneyland. Ya slightly larger than a parking lot. In that decade time they could have built new studios or something. The average new AAA game studio takes over 10 years from concept to first game. Sony just didn't have all these dev teams out of thin air, it takes much time.

You aren't factoring in people who own neither a 3DS or Wii U that may buy at least one NX sku.

Lets use the 2015 releases as an example, lets say someone wanted to play Monster Hunter 4, Majora's Mask 3D & Happy Home Designer but those 3 games weren't enough for them to buy a 3DS, lets say they are also interested in Splatoon, Super Mario Maker & Xenoblade X but again those 3 games aren't enough to buy a Wii U. Would that person be more likely to purchase a single device that plays all 6 of those games?

Another factor that you aren't considering is the increase of software sales, certain games have been hindered by the fact they are stuck on a platform with a small userbase. Splatoon & Mario Maker are big successes and will each sell 5+ million but what if they had the entire 3DS audience to sell to as well? These games could very well sell over 10 million.

A third major factor is that Nintendo can diversify their lineup by not having to make 2 separate entries of many franchises. Instead of releasing a Mario Kart title for the handheld then 3 years later another Mario Kart for the console, they can release one and support it with DLC then release a new IP 3 years later. This potentially leads to more Splatoon-like scenarios of new, exciting franchise being created which increases the devices appeal.

Thats why I mentioned it.