"Poo slaves" are not dying specifically because their numbers are less than amount of yukkuris that are crapping into their hole. Or in more rare cases yukkuris that support them are very well fed, and produce large amounts of poo.
If one could indefinitely survive on it's own poo, that means that same amount of poo would keep coming out, never lessening in the cycle. That kinda defeats "poo slaves" purpose, they would crap out same amount that they ate. Never actually helping with recycling or prolonging use of the hole.