The post said there will only be 15-20
NPC sororities by 2015. NPC and national are not synonymous.
Even if a sorority elects to leave NPC or falls below the minimum requirements, they can continue to be a national sorority. They don't have to close anything. That is what Kappa Beta Gamma has done - they never died out completely, they are still here.
I think that with multicultural groups and social/professional options, women are more open to joining a non-NPC group than they used to be, especially at smaller schools.
Remember the threads about Kappa Sig & Phi Delt leaving the NIC? I wonder what would happen if one of the 26 left the NPC voluntarily, even though they were strong. That would be interesting to see.