The same argument can be given for pets, most of them will not survive in nature anymore. But that can't be the argument to continue zoo's (and to continue holding pets for that matter). Before we had the internet, before we had magazines, a zoo may have served the purpose of teaching the public about all sorts of animals living around the globe, next to being entertainment. By now, I believe the teaching part can't be the argument anymore to continue zoo's.
Holding animals in a zoo, captive in a small-ish geolocation, behind fences and gates, isn't that much different to holding pets in and around our own home. Same same in that respect.