I'm don't think it's quite fair to mark up every mixed-species enclosure death to an "active human decision". Even though we're talking about mixed-species combos that may have ended in death, zoo animals have sometimes killed members of their own species, or have killed wild animals that wandered into their enclosure. Death is a part of life, and for every mixed-species combination that has ended in death there has probably been more than a hundred that didn't, and probably the vast majority of those have been successful; otherwise, walk-through aviaries and coral reef tanks wouldn't be ubiquitous. I also doubt that most zoos have a "blase" attitude about mixing species; on the contrary, from what I know a lot of precautions are taken before doing so, from exhibit design to gradual introductions and constant monitoring.
There was an okapi and an antelope (a nyala, or maybe kudu?) that lived together at the Blank Park Zoo in Iowa. They lived together for years, were close and liked to play with each other... until the antelope accidentally gored the okapi during play and it died of the injury. Was this foreseeable? Not really; technically it was always a possibility, but never a likely one... and up to that point, the two animals greatly enriched each other's lives. Tragic accidents happen, despite the best efforts and intentions of those involved.
Anti-zoo groups will use anything as ammunition

It seems to me that avoiding
all risks and only doing things that cannot be misconstrued by anti-zoo media campaigns is both a waste of time and a better recipe for poor husbandry than mixed-species enclosures are.