Congo is, of course, a complete mess. You're taking the worst, most difficult example and trying to extrapolate from there. The situation would be far better in Zambia, Rwanda, Uganda or other surrounding countries. Even in the Congo, I doubt that the numbers are that over 50% of the mosquito nets are used for fishing as you claim.
People do study the effectiveness of different approaches. It doesn't take a genius level intelligence to go through a year later and check that there were fewer reported cases of malaria.
Obviously nets can be used for fishing. It may be news to people watching TV documentaries in the US, but it's not news to anyone who has visited Africa.
There are other approaches such as spraying the inside walls of every house with insecticide. People are doing this in some areas. It's probably more effective and there is nothing to steal. The advantage of mosquito nets is that you can do it one person at a time. But with the insecticide approach you want to try get every house.