Anti poaching patrols do need to be paid for. Selling an old bull for hunting is the most direct path to gathering up such funds. There is actually a fairly large population of white rhinos in private ownership in RSA. And current regulations make it very difficult for those owners to pay the costs associated with owning them, much less any profit. Dart hunts have become much more difficult. I have heard first hand of such owners reducing their populations because 1. They can't cover the costs of ownership. 2. Ownership of rhinos is very risky. 3. Both the above come together to make such ownership a loosing proposition.
Poaching really escalated when the international horn trade was stopped. My observations of the RSA game farming industry is that if those farmers were allowed some freedom of trade in all rhinos and their products, but especially the international horn trade. And no future import restrictions on hunting trophies. Within 20 to 30 years there would be more white rhinos in RSA than anyone would know what to do with them.
As it is now, it seems to be a large industry of "save the rhino". Huge amounts being donated/collected. Large amounts going into advertising for more funding, poaching cartels reaching into many levels, and piss poor results.
If you have the opportunity to hunt an old bull, go for it and be proud your doing more for the population than most anyone else.