Is that something Nintendo ever said or suggested? I only remember seeing stuff like that from hopeful, starry-eyed forum dwellers.
They did say things like presenting a device that appealed to fans of different form factors across regions.
They did say they want to create 'brother systems' with a shared platform like iOS.
They did unite their hardware departments and their software departments.
They did announce a hybrid console and announce games absent on Wii U but present on 3DS for it. FE. SMT. SE JRPGs.
They repeatedly show it as a portable, both handheld and tabletop.
Japanese developers have mentioned it as a portable for Japan to shore up PS4 sales overseas.
...
If there is a 3DS successor that competes with Switch for software from Nintendo and Japanese third parties, that is a colossal risk to the Switch and is a butchering of the hybrid and shared libraries concept.
If it exists it had better be a new Switch model or something that plays games smaller than 3DS games, but it's hard seeing such as that compete with mobile.