Other than Virtual Boy and CD-i, has Nintendo ever given us crap as a console? Each one of them had a selling point, whether you agree with that selling point is pure opinion. I believe that their next console/handheld if they saw a need for unification, we'd see it. But I don't think Nintendo sees a need for that or even if it's possible. However, I see them doing a unified experience of which each console game has a handheld counterpart much like Smash Bros. has done. That way, you can choose or have both and play the games at any place in time. I always anticipate their next generation because they're always going to do something we haven't seen.

Another issue is reduced sales. If they go with this and all games are usable between the devices. I'm not buying the handheld. I don't care about playing the games outside. This type of loss has to be considered.

unified handheld console will never happen.

Graphics don't matter. It's all about gameplay. It would at least sell a billion because of that. Nintendo is King of Gameplay.

Hardware isnt going to cut it. Nintendo needs to become 3rd party. Otherwise they will bleed the money they made slowly.

I think the handheld and the home console will be one generation apart in power, and they will have the same games, the difference is that on the home console the games will look like The Last of Us for PS4, and on the handheld like the Last of Us for PS3.
So, Nintendo will only make one Mario Kart or Smash Bros, not 2 (one for handheld and one for home console), and they will dedicate the free time to making new games.

I'd buy both. The ability to for your games to go to both devices makes it too sweet to pass up. 3rd party devs won't have that excuse to just develop for the handheld anymore. I mean their console version won't be the prettiest if they don't want to maximize what the console can do... but it will lead to a giant library for both at the same time. And since Games is the big issue for Nintendo this generation, it pretty much solves all problems.

They can reach those kind of sales if they play their cards right.
If they streamline their games production that will not only allow them to fully support their consoles - with even a game per month! - but they will also need new franchises to fully support the consoles throughout their lifetime. Which could and should mean new franchises like Halo, Gears of War, Uncharted, Gran Turismo, Demon's Souls, etc.

With the extra staff they can develop bigger projects and more ambitious projects.

Another factor that could also help sales of the console is if Nintendo bundles the home console (just the console itself) with the handheld, for 350-400€/$.
That would really attract a lot of customers because of it's incredible value.

Also, if Nintendo follows the above path, a special retribution system for game purchases for those who own both consoles, would be a very good incentive for people to buy both.

Depends on the execution.

if Nintendo does release a unified handheld/console platform next gen, the sales won´t be  3DS + Wii U, because too many people have both: 3DS and Wii U. If Nintendo create a hybrid unique console, it´ll be one sale to these people, not twuo sales (3DS + Wii U).

Nintendo is in real trouble.