Your argument seems to be that space should be devoted to species other than tigers that have greater conservation value (as noted above, I do not believe that there are excess tigers of the three managed susbspecies in AZA care, so I don't believe your point that there are too many tigers in captivity).
Since ISIS stopped being generally available on-line, it has become more difficult to check on the number of zoos keeping different species and subspecies of animals and the number of individuals contained. Most endangered species do not have viable captive populations, so are at far greater risk of extinction than those that do.
I used Zootierliste to provide a list of zoos keeping different species and subspecies of cats:
Eurasian lynx 389 (Carpathian 37, Central Asian 5, Eurasian 122, northern 196, Siberian 29)
Tiger 385 (Malayan 10, Siberian 178, Sumatran 51, tiger 146)
Lion 359 (Asiatic 43, Barbary 22, Kalahari 4, lion 225, Somali 1, Southeast African 45, Southwest African 19)
Leopard 232 (African 12, Amur 54, Arabian 3, Caucasus 1, Central African 2, Indian 1, Javan 3, leopard 59, North Chinese 29, North Persian 46, Sri Lankan 22)
Wildcat 179 (African 3, Caspian steppe 1, East African 4, European 134, Gordon’s 11, Cheetah 132 (Cheetah 1, South African 115, Sudan 16)
Serval 118 (Serval 117, Southern 1)
Jaguar 103 (Amazon 5, jaguar 98)
Snow leopard 98
Cougar 96 (Chilean 5, cougar 89, Missouri 2)
Ocelot 60 (Central American 1, ocelot 59)
Caracal 50 (Caracal 26, common 21, Schmilz’s 2, Turcmenistan 1)
Leopard cat 45 (Amur 29, Indochinese 11, leopard cat 2, Palawan 3)
Fishing cat 40
Pallas’s cat 37
Jungle cat 34 (Jungle cat 29, Palestine 5)
Indochinese clouded leopard 31
Geoffroy’s cat 29
Sand cat 25 (Arabian 25)
Jaguarundi 24
Bobcat 20
Margay 14 (Central American 1, Yucatan 13)
Rusty-spotted cat 13 (Sri Lankan 13)
Asiatic golden cat 9 (Chinese 1, South East Asian 5, Tibetan 3)
Canadian lynx 6
Oncilla (Oncilla 4, South Brazilian 1)
Palestine 4, Scottish 22)
Iberian lynx 2
I was surprised about the number of zoos keeping Eurasian lynxes. I also produced a list of mammals represented by species and/or subspecies in over 250 zoos on Zootierliste. This is only a rough list, as some zoos keep more than one subspecies or variety of the same species:
Fallow deer 617
Red-necked wallaby 450
Red deer 431
Wolf 401
Northern raccoon 394
Eurasian lynx 389
Tiger 385
Meerkat 375
Mouflon 363
Lion 359
Ring-tailed lemur 339
Ring-tailed coati 326
Brown bear 298
Plains zebra 290
Eurasian wild boar 269
Giraffe 263
Once again, I found several surprises as regards inclusions and omissions. Several of the species are not endangered and some are often kept in parks and could be replaced by other species. I did not include domestic animals, but many of these are kept more for their popularity with visitors, rather than any educational value.
Except for very few cases, the answer does not seem to be removal of individuals of critically endangered species from the wild, at least not for ex situ exhibition. Even the species you note (the Mauritius kestrel and the echo parakeet) were managed in situ at first- there was not an attempt to set up breeding groups at institutions outside of Mauritius until the population had been stabilized through in situ work. For an example of how ex situ breeding can utterly fail when there are not sufficient founders, one need look no further than rhinos: both the northern white rhinoceros and the Sumatran rhinoceros failed as captive populations.
I accept that it is better to protect endangered animals in the wild by protecting their habitat. When I visited Madagascar, I saw an enclosure that had been created around an area of trees to keep indris, prior to exporting the indris to Paris. All the indris died and the experiment failed, as it did with most of the Sumatran rhinos that were captured from the wild. I do not advocate capturing other species if they have a similar fate. I know people who are involved with saving species in the wild, but I also feel that zoos could allocate more spaces to have back-up captive populations. As wild habitats will continue to be lost over the next few decades and the human population will continue rising, it is imperative that as many wild habitats are protected now, if people want to conserve large species. Keeping several hundred individuals of some large species in zoos, while ignoring smaller species, is not conservation. Too many species have died out and will continue to die out.
Finally, your argument seems to be that zoos should not focus on species that the public wants to see; to extrapolate, it almost seems that you argue that captive breeding should be the most important function of zoos. Why not get rid of the vast populations of non-breeding animals maintained solely for display, education, and research programs? I must admit, when I was younger, I used to believe that zoos should be focused on the propagation and rescue of endangered species, but I have grown to strongly feel that, as much as I love zoos, their biggest contribution towards conservation is education and ex situ field work.
I do not expect you to agree with me on this, but I would like to have more of a balance in the animals that zoos choose to keep. Keeping animals purely for their popularity is not conservation. I remember zoos that had several species in small enclosures and do not like seeing animals in barred concrete blocks. While i can understand why animals should be kept in relatively natural conditions, I believe that 'all animals are equal', but not that 'some animals are more equal than others.' Land for a new tiger enclosure could be used to keep several smaller carnivores, which could educate people, as well as helping conserve the species. Does any zoo need 3 meerkat enclosures, when many mongoose species are poorly represented? Is any visitor really keen on seeing the same species several times in the same zoo?
Some of the most popular animals are not endangered. Many visitors are interested in leaf-cutter ant colonies, even though the ants can be pests where they live. Similarly, some endangered animals can be boring when inactive - the giant panda is one example. It is a case of having a balance and realising that zoos could keep thousands of species of animals that could intereet people in the natural world.