Under the current scenario you have (a) intelligent middle class and wealthy that breed at capacity levels (2) or below capacity level (1). (b) Intelligent poor people that don't breed (0). (c) Stupid poor people breed beyond capacity levels (+2). Stupid poor people are subsidized by middle class and wealthy people through welfare and other programs. If subsidies were cut I think the birth rate would remain the same for (a) and (b) but the post natal deaths for (c) would rapidly increase.
To improve the situation for (b) would require subsidizing. I'm reading a book
right now and this is exactly what was done. Karaits overall didn't hold together very well but in some periods they had an internal welfare system that paid for food, school, health care, ect. If you watch A Life Apart
you will also see an internal welfare system done by Rabbis.
(b) can survive but they need to join a community that will subsidize them.