This is a great question! I think a key reason for these failures is how many species were imported to multiple facilities around the same time, only for numbers across the wider gap to die out, often persisting in only a few key institutions, as happened with River dolphins and Sumatran rhinos. I think it would be better to import a few individuals to a single facility that is prepared to make a long-term investment in the species and is willing to coordinate with ex-situ programs to further research - and if that facility can succeed, the research can be extended elsewhere, versus importing the species to a dozen locations and hoping they can all figure things out.
Not to mention that when the animals get spread out, it means that each facility gets fewer animals. So when one animal dies (no matter the cause) the facility may be more likely to cut their losses and give up. Even if they want to continue displaying the species, getting more of them might be too difficult. If a place has 20 rhinos and one dies, well, they still have 19 rhinos. That's easier to deal with than a place losing one of two rhinos.
I agree with @vogelcommando that it could be helpful to use rescue animals to try and develop better captive care methods. (if nothing else, it would be beneficial for future rescue animals) I for one think that the pangolin would benefit greatly from zoo display and captive breeding, if zoos can figure out some more practical ways to keep them.