I think some people are missing the point about conservation that it's not just about keeping a particular species alive, it's about the positive downstream effects of releasing that animal into its natural habitat - releasing a panda for example has huge benefits to the surrounding forest as they act like pollinators, dispersing bamboo seeds for miles around them which also benefits other animals and can help promote or inhibit other plant growth. This can also positively impact waterways and benefit humans too.
The wolves of yellowstone are a well known case study of the conservation of one animal having huge benefits to the surrounding environment.
Funding can be a major issue for conservation, zoos are one avenue for running the right breeding and release programs. Zoos can also be educative and prompt people to donate to relevant charities outside of the zoo itself
I do agree that there's a large quantity of zoos that do not prioritise the animals' welfare or provide any tangible benefits to conservation. However there's also a host of very responsible zoos where the animals seem well looked after and there is adequate enrichment and space provided for them to act more naturally.
Having said that I don't think big cats and great apes are well suited for zoo life - I've never seen a happy lion or tiger in a zoo and wish they'd be given larger habitats to live in with a more natural mix of animals (basically a wildlife reserve)
Conversely, I've never seen an unhappy meerkat- some animals are better suited to zoo life and seem to actively enjoy it. The penguins at edinburgh zoo for example seem happy to me and I regularly go to watch them dip and dive in their massive swimming pool