Barring an absolute failure on the Switch's part, yes, the Switch is replacing the 3DS. Half the point of the Switch was to consolidate their handheld and home console software and hardware teams. They internally reorganized around this. The idea being to focus only on a single platform, increasing the number of unique games due to no longer having to develop completely separate games for two different pieces of hardware. No more Mario Kart 7 and Mario Kart 8 or downports of Donkey Kong Country Returns and Hyrule Warriors to the 3DS, just "Mario Kart 9, playable at home or on the go!", etc.. Nintendo has already said this was a goal that inspired the design of the Switch, and we know for a fact their handheld and console hardware teams merged a few years back. So no, it's not gonna be bogus.
The only reason Nintendo keeps presenting the Switch as a home console and not a 3DS successor is the same reason Nintendo pretended the DS was a third pillar and not a successor to the GBA. It's to save face in the event it back fires and to have an "out". So sure, if the Switch fails miserably we may get a 4DS announced for 2019 or something, but if not the 3DS will go the way of the GBA - quietly phased out as its intended successor takes over entirely. Keep in mind, even for the GBA that took two years.