the thing is, Fusion doesn't need to outsell WiiU/3DS combined(most likely 80+ million). It just needs to have a large enough install base for Nintendo's IP to sell on and to make a profit.
Having a unified handheld/console will make hardware R&D costs significantly less since they are essentially making the same hardware just in different casings instead of 2 completely different devices.
it will also significantly help Nintendo's internal software output. We are already seeing it this gen that Nintendo struggles to give strong support to two seperate pieces of hardware at once. this will only get worse next-gen if Nintendo keeps their hardware seperate. Nintendo's next handheld will likely be in HD as well which will make software for it much more expensive and time consuming than 3DS software and if the next console is somewhere between PS4 & PS5 in power then dev time/costs will likely be much higher than Wii U.
this type of scenario would likely see both Nintendo platforms have awful 1st party support which would lead to poor sales and most likely losing a lot of money. honestly I can't see anything other than a Fusion platform being Nintendo's next step.
When the herd loses its way, the shepard must kill the bull that leads them astray.
