Because Nintendo always had low price and high price point offer on market, Switch with its price point of $300 is high price offer, 3DS with $200-80 is currently low price offer, Nintendo handhelds always had low price point and thats why they were selling like device per person (for instance family with more kids will much more likly buy several Nintendo handhelds for each kid if price point is low enugh), and when 3DS dies Switch will still have price point that will be above $200 (probably around $250), they will need $150-200 Switch offer when 3DS dies.
Less parts, less tech, smaller form factor, package...means less costs. Of Course that smaller Switch, smaller factor size, smaller screen, without dock, built in controls instead of detachable Joy Cons and whithout some features from Joy Cons, whithout parts need for playing in docked mode, HDMI cable, smaller package...all means much less cost. So yeah, I think that all that could cut down cost for around $100 compared to regular Switch.
Switch Mini/Pocket just for handheld play like real replacement for 3DS price point just make too much sense, and I think we will see something like maybe even next year.
Because Nintendo eventually wants Switch to become a one per person system along with attracting more kids, families & budget conscience consumers.