If you think about it, I am also convinced the Switch will be the real 3DS successor, however... Nintendo didn't seem to be that convinced in the first place. Just look at their shipment numbers, they shipped enough systems for a WiiU-Successor but by far not enough for a real 3DS successor, they had to double production within the systems launch week to at least satisfy demand to some part.
It's a bit sad to see it this way, as it shows that Nintendo had lost faith into their fans with the WiiU, it's understandable though, the WiiU bombed big time and cost Nintendo dearly, they produced lots of systems that just wasted away in storage facilities and store shelfs. Sales wise I fully expect the Switch to sell at least the amount the 3DS did, probably much more and perhaps in reach of the Wii or even NDS. Only time will tell and we'll see if Nintendo puts all their effort into this one plattform, if they do, I don't have any reason to not expect the system to do very very well.