I think there are a lot of zoos that do it right, and zoos that do it wrong.
Here in Aus there are some amazing open plain zoos that I wouldn't consider to be cruelty. Then there are zoos that take care of their animals well, but that have enclosures that are too small. Then there are some zoos that have animals in terribly small enclosure, with terrible conditions etc.
It's almost like people with bettas, there's good, there's bad, there's average. I agree that a lot of zoos are involved in conservation and breeding, and when you consider how threatened some animals are now, these zoos may soon be the only thing saving them from extinction (eg there are now only about 30 mountain gorillas in Uganda now, and rhinos are incredibly threatened in other parts of Africa). Some species of tigers now have numbers in the wild less than those in zoos etc.