Environmentalism is anti-humanism.
They are discussing offshoring this sand to other countries that can afford it, not to build useful African structures. An argument from "The world has already been using this for a long time" could be used for any number of grotesque and terrible acts (and has) so I will ignore that part.
The main complaint about the "rivers dying" is that it has a outsized impact on the poor who use those rivers for food and water (which is no longer available and which is no longer captured, respectively.)
While some people truly believe in the preservation of nature over humans, most of us "Environmentalists" want to save the environment because of the direct and indirect human suffering inappropriate exploitation of our shared resources cause.
This is a similar problem. When you dredge up some sand and a river disappears, you've made a mistake that has human consequences and it probably was not worth whatever you did with the sand.
I see. That must be the reason why Americans are starving and Africans are so fat because of too much they grow on their undesertified fertile farmlands.
Let's assume your points are 100% correct. If you make them by ranting, like you've been doing in these threads, you not only break the site rules, but discredit the truth. That's not in anybody's interest.
All you have really proven is how much you dislike environmentalists. A debate that is quite colloquial to a few cossated western countries. In a lot places people actually live within touching distance of the environment. The pros and cons are just obvious to everyone. Young local men get paid to move sand, which is good. But farmers need water for crops and erosion is a problem. It becomes news outside because no one has a good solution. Just another intractable social problem.