What?
The purpose of mixed-species exhibits is to add living enrichment for animals and display what true ecosystems could look like in the wild. What do you mean they rarely worked well? Have you never seen or heard of mixed African exhibits, aviaries with multiple species, Orangutan and Siamang? There isn't really competition in zoos, some animals might want to get food first but in the end, it doesn't affect their general food status so they couldn't care. Good zoos do give mixed exhibits a large amount of space, also good zoos don't keep large mammals in cages so don't use that to try and prove a point.