The NES Classic Mini was meant to be a timed novelty item; so was the SNES Classic Mini (though Nintendo claimed to have learned from the incessant demand for the NES Classic Mini, and promised to have more in stock this time around).
Apparently, the sheer demand for these systems has changed Nintendo’s mind on the status of these systems, however- in a new statement, Nintendo has confirmed its intent not just to continue selling the SNES Classic Mini beyond the end of this calendar year (which was when it was originally slated for discontinuation), but also to bring back the NES Classic Mini next year, too.
The NES Classic Mini’s availability was a total mess, with the system being produced in extremely short supplies, and being sold out, then being flipped on auction sites for exorbitant prices. Nintendo has previously emphasized that the SNES Classic Mini would not be a repeat of that, with Nintendo’s Reggie Fils-Aime very explicitly urging would be buyers to not resort to buying from scalpers.
The exact date that the NES Classic Mini will return to being sold next year is unknown for now- Nintendo has promised to share more details on this on a future date.