Yes, Nintendo Switch is the successor.
Some 3DS owners have already fully replaced their handheld gaming time with Nintendo Switch.
Others are waiting for something cheaper and smaller.
I see the Nintendo Switch Mini/SP/(whatever they call it), launching for $199 around October 2019 just before "handheld focused" 2019 Pokemon arrives.
That synergy (of cheaper/smaller hardware + Pokemon) would capitalize on the opportunity to transition a lot of 3DS stragglers, as well as, being the hot Christmas item for parents to saitiate the Pokemon fevor of their little ones.
The most likely reason they aren't doing it this year, is because they want a better cost margin on the SOC die shrink (probably 10nm or 7nm) that will be cheaper next year, AND that Gen 8 Pokemon won't be ready till next year anyway.
Really, if they Ace the Design, Marketing, Ample Supply of Nintendo Switch Mini, and the Quality of Pokemon 2019 is High, then this could be Nintendo Switch's "DS Lite moment" and the platform as a whole could see a huge boost to hardware/software sales long term.
