It ultimately comes down to the choices Nintendo makes for the next devices, they made a few pretty big mistakes at the beginning of this generation that probably cost them some sales.

3DS and Wii U were the most expensive handheld/console they have ever released, they both suffered from customer confusion by thinking they were simply a revision/add-on for DS/Wii, Nintendo has had pretty mediocre marketing/advertising this generation, and both have suffered from multiple droughts between big releases.

If Nintendo can fix those issues while not making new ones then it's possible that Nintendo hardware can see an improvement in sales with their next devices. Also due to the fact that they would no longer have to make 2 versions of all their franchises, it opens them up to making new ip at a faster rate which gives them a higher chance of creating the next big Pokemon/Skylanders/Minecraft craze.

40,000,000 for me, close to Man Bear Pig originally prediction with Wii U.

what is your definition of fusion?

if the Fusion seriously is a successful weird connection of the home console and the handheld that is essentially one and the same, then yes, it will sell over 80 million. however it is hard to see Nintendo successfully merging their consoles and handhelds at this point as the technology seems totally far off

Not really, it's completely possible for Nintendo to make a handheld that is able to produce Wii U level visuals at a lower resolution, similar to the way 3DS can handle Wii level visuals at a lower resolution. Then for the console, they just make a souped-up version of the handheld that is more powerful than Wii U and plays all the same games at a higher resolution with some extra graphical effects.

I wish i had psychic power............ 30 or 50 million.

I don't think so. If Fusion will turn to be a handeld+home console, I see very unlikely that it could sell the same as WiiU and 3DS combined. I presume it would be a 50,60 million seller.

If this is the real path Nintendo is going, they really don't read the market very well. I mean even the best case scenario sees the popularity of their handhelds lifting the home consoles by 10M+. But, that also means the lack of popularity of their home consoles is dragging down their real money maker, handhelds. In other words, not going to happen. And there are 2 really big reasons why this will happen.

First of all, the handheld market is shrinking. This is coming from someone who didn't want to believe it early this gen. But, it's true. Even the successful 3DS is going to struggle to hit 80M+. What chance does a handheld that you have to buy a home console with? Very little.

And, most importantly, the Fusion will be another Wii/Wii U in terms of power. Unless Nintendo wants to price it at $599+, since they really can't afford to have another HW taking losses at launch. This means that unless Nintendo wants an outrageously priced home/handheld system, we are looking at both systems being a little more powerful than the PS4 (maybe less powerful  if they want a really low price.) That might be good enough for Nintendo fans, but it's not for the mass market. Not when the PS5 will be launching a few months to 1 year later and is probably going to be 4x-5x the power of a PS4. We're talking true CGI graphics here.

I think you're confused about the Fusion concept. 

Why would you have to buy the console to use the handheld? 

You don't have to buy an iPhone to buy an iPad and get use of all the same apps. 

Depends on what rumor you listen to and how hard Nintendo wants to push the fushion idea.