The whole ban on even-toed ungulates seems a bit too "blanket", if you know what I mean, and doesn't take in differences between species. I would like to know, for instance, just how dangerous disease-wise a pygmy hippo is likely to be to our grazing industry.
Getting back to bongos, I believe that without fresh blood we should attempt to maintain the species as long as possible. Their close relative the sitatunga (which in our region is just about gone - 2 males 6 females in New Zealand; 2 females in Australia) has lasted for 50 years after just one importation and would probably have continued for another decade or two with proper husbandry ( they were "let slide" because the bongo was the preferred species - how ironic!)
It's worthwhile keeping the bongo going because somewhere in the future the situation may change. For example, cattle embryos are routinely imported and implanted in surrogate mothers - maybe the same can be done with various tragelaphine antelopes using elands as surrogates. It won't happen in my lifetime, but may be possible in the future.