I believe that Dassie rat and I have more similar viewpoints than it appears.
More precisely, when talking about tigers, elephants, gorillas and other wide-ranging mammals, I was referring to In situ conservation, and the preservation of habitats.
Habitat conservation is the first and foremost goal here, and I see that we agree on that.
The tropics are incredibly rich environments, and zoos cannot hope to preserve even a fraction of the biodiversity that is present in those habitats.
For example, Indo-Burma has something like 13,500 species of plants (7,000 of that endemic), 433 mammal species, 1,233 birds, 522 reptiles, 286 amphibians, 1,262 freshwater fishes, and God knows how many species of insects, fungi, bacteria etc. (source: Conservation international
Biodiversity Hotspots - Indo Burma - Unique Biodiversity )
For comparison, Europe has 270 mammal species, 800 bird species (445 breeding), 123 reptiles, 75 amphibians and 344 freshwater fishes.
Alternatively, we may compare bird species numbers: 3,200 in South America, 2,900 in Asia, 2,300 in Africa, 2,000 in North and Central America, 1,700 in Australia and the surrounding islands...
Another example is the biodiversity of the Amazon: the basin of that river alone has more species of fish than the entire Atlantic ocean.
However, when we talk about In situ conservation, preserving charismatic, wide ranging animals like tigers, lions, elephants and gorillas is not an unnecessary drain on the resources; quite the opposite, it enables saving a whole range of species that share the same habitats. An average Asian or African rainforest has probably more species than all of the Pacific islands combined.
On the other hand, if we talk about Ex situ conservation, I also agree - unlike ungulates, fish, amphibians, birds or most other wildlife, large predators are almost impossible to return to the wild. Their maintenance is expensive, but on the other hand they are a major attraction of any zoo, and as such in many cases contribute towards fundraising for In situ conservation.
One solution would be to preserve as much genetic material from different individuals as possible (it is already done in a few facilities, so called "Frozen Zoos"), but a certain number of live specimens will always be necessary, as those samples, if they are ever used, will need a host.
edit: as Stefka said, it is still much easier to protect a rainforest than to stop ice from melting.
Giving up is dangerous, it just means that it will be easier to give up with the next, less well known species in line, and that conservation funding will gradually diminish.