Errrm... No, a 3DS successor wouldn't be unreasonable, at all, and it wouldn't show a lack of confidence in the Switch. I would go further, and suggest that it should happen.
The Switch has reformed Wii Remotes in the detachable Joy-Con. That's important to preserving the Wii legacy, and making that library accessible in future (over 1500 games), whether it's with VC or HD remasters. It's been advertised as a home console, and you've been told as much from the horse's mouth, but some of you have been telling yourselves it's a handheld to make yourselves feel better about specs you don't even know, if not to lowball something that's more capable than you think. It's portable, but it's still a home console at heart. I've used the expression "laptop console" compared to the X4's "desktop consoles", and perhaps that is the most accurate description, because you can have the console experience without being shackled to a TV screen and four walls. You can see examples of that in the Switch reveal clip.
At the same time, the DS is their best-selling platform, while the 3DS passed the PS2 total sales in Japan recently, and saw an upturn in hardware sales - It's not a stretch to believe that a dual-screen successor will exist in their future, or that the (3)DS legacy will be preserved, too. That's a combined library of about 3000 games. It could build on the Switch concept - perhaps a dual screen device with 360-degree hinges for separate screen tabletop multiplayer. You could add the Wii U library to that, too. For a site that claims to know so much, it's quite amusing to see how many people would believe that they're done with the ideas behind their most successful products.
It would be a smart decision to retain those gaming concepts. The over-reactions in this thread are a result of people falling into the trap of presuming that a 3DS successor would mean a separate library, when it would most likely mean a shared gaming library across a range of home and portable products (similar to what Apple has with the iPhone, iPad and iPod Touch), or a lower point of entry, or improved battery life - the trade-off would be that a Dual-Switch would play their games in portable mode alone. It's not so hard to imagine the possibilities, and perhaps it would be consistent with the vision that Iwata presented before his death.