@forumbully: You could ask the same question in regard to many animal species which are nonetheless still kept in zoos today-like Great Apes, penguins, antelopes...
I would estimate that about 20-30 botos were caught in the previous decades-a rather small number compared to the estimated amount of specimen regularily drowing in fishing nets or being killed deliberately by fishermen. I think a lot of the losses especially by the American zoos can be contributed to incorrect catching, transportation or husbandry. Reading Gewalt's accounts of the catching, transportation etc. lets one realize how little was known in the past about these important issues and how "inventive" the collectors had to be due to problematic and "primitive" conditions.
Back then, Gewalt animadverted the reluctance of his coevals to try out new paths and thus gain knowledge about animals that could prove to be important in the future. Therefore, I consider the husbandry of botos over decades at Duisburg Zoo not as an odd exception, but as a great waz to learn more about these animals and as proof that it is possible to keep and, as the case of Venezuela shows, even breed river dolphins in captivity. And in regard to the situation of the river dolphin species worldwide, I do think that one should not easily scrub the possibility of, additionally to better in-situ protection, keeping some ex-situ populations of highly endangered river dolphins like the Ganges River dolphin. Your hint re: better international collaboration would be of high importance here, too.
The botos at Duisburg have always been very popular among the visitors, especially after "Father" and the still living "Baby" moved to their new exhibit. And judging from the videos and travellersˇs accounts, the ones at Venezuela seem to be popular, too.