Sure, I could see them releasing a SKU of the Switch that is pocketable. Where both switch and switch pocket play all of the same games.
But a new handheld with games that can't run on the Switch? That would never sell now that the Switch will be on the market.
And, It would have to compete against the Switch and 3DS with too many disadvantages.
The issue is price and development costs. N3DS XL is priced at $200, Switch is priced at $300, a next gen DS with even more advanced hardware would be priced at $250 or more. Developing for the switch covers both a home and portable game in the cost of developing once, developing for a seperate non-compatible system would only pile on more costs to develop first party games for both systems.
They have cornered themselves by having a product like the switch that covers so many advantages and uses. TV out/splitscreen/controller options/Switch will probably be less than $250 by the time this non-switch thing could even launch/Exsisting game library by that time as well/etc.
As price drops on the Switch, they will only be able to launch a new handheld that exceeds all of the advantages of buying the switch.
The obvious being cheaper price, pocketable, and longer battery life. 3DS already covers those advantages.
Once they can shrink the die to ~10nm or less. Then they can produce a Switch mini or clamshell SKU or whatever is pocketable, with the same internal specs that plays the same games. This allows for more advantages over the switch and would justify a purchase and could be considered a dedicated handheld, all while minimizing additional hardware and software development costs.
Other than that, Nintendo could release more revisions of the 3DS and try to maintain that market, but they are not going to make even more hardware that requires another totally seperate game library, branding, development, etc. until both 3DS and Switch have have both had a very full life cycle.
With all that Nintendo has communicated about wanting software compatiblity across many devices, the switch serves that role. It can be hybrid hardware, dedicated home console hardware, dedicated handheld hardware, etc. same game library, same OS, different form factors, different hardware SKUs, etc.