There isn't just gorillas at zoos though. Big cats who would wander miles in a day confined in a small space is cruel.
Did you read the article were in mentions pacing and head nodding being common place at zoos and signs of an animal in distress.
I like zoos and have been to many, and there is a wide spectrum of quality.
Some zoos you can tell prioritise the visitors and entertainment thereof over the welfare of the animals. I couldn't say if any zoo prioritises the animals COMPLETELY over the visitors (it's their income after all), but some zoos do seem to have strong preservation, educational and conservational approaches.
But just for example, I've seen a lot of animals sit in the corner and butt heads on the wall, or gate. In another zoo, they had a train that went around the park, with a loud bell sounding off every five to ten minutes to signify the train leaving....right next to an enclosure where a leopard was trying to sleep. I literally stood there for half an hour and watched that leopard wake up every time the bells went off.
Ok, so maybe "in the wild" large cats probably have to deal with all manners of noise and what not, but you know, I'd argue whether a routine bell would be one of them and whether being in the presence of such is a "happy" experience.
That said, even the notion of happiness can be biased towards our own definition of the word.