If the NRA folds (and that's a big "if," since it may have been exaggerating its state for the court), I could see at least a few outcomes.
First: a similar group pops up, probably bankrolled by fresh investments from the gun industry and right-wing backers hoping to use gun rights to keep Republicans loyal. It could reignite the NRA's base, but it'd also face the same limitations: many companies wouldn't want to touch it with a ten-foot pole.
If not that, then it'd be down to the ACLU. Which would drive conservatives nuts, because the ACLU may defend the 2A on a core level, it's not the absolutist "any gun I want should be legal" kind of defense the NRA's hardcore members prefer.
The least likely: a new group arrives that casts off at least some of the corruption and fanaticism of the NRA in order to find financial acceptance. I'm really not expecting this one, because it'd neither keep the zealots nor get a mainstream audience.
As a side note: isn't it funny to think that at least some of the NRA's current predicament was sparked by teens? Gives you hope that the country's current nightmare isn't going to last long.