So yes, Sumatran rhinos have had limited success and only in the last 10 years. But its not over, unlike what you seem to be suggesting. I understand what the limitations and realities are for captive breeding - you just don't seem to want to accept it yet.
I hope that it's not over. I want a world with Sumatran rhinos.
Watching the Sumatran rhino program launched with much fanfare and then fizzle over the course of decades, from the perspective of the attempt to establish an American zoo population, has been very painful. Combined with what seems like unending grim news from the field about rapidly contracting population numbers (improved census techniques or not, the trend is downwards). I don't see how an inbred population of a few Sumatran rhinos in zoos would be helpful for their conservation, but you are correct that this was the starting point for condors, black-footed ferrets, etc.