In the bad old days, a zoo often acquired a pair of animals, preferably unrelated, bred them every year and traded the offspring on until one of the parents died. That still happens with some species, for example the aforementioned maned wolves, but at least now the studbook system allows the stock to be managed sustainably by making it easier to re-pair animals that have lost partners and to pair up young stock.
In the case of species like the lion-tailed macaques, they may be held by fewer collections, but the sizes of the groups are much larger now; so there are now probably significantly more specimens in the country. Of course these larger groups need more space, so the older zoos now all hold fewer species than they did twenty or thirty years ago.
Several species have been phased out to make space for new ones, either by exporting the animals to collections overseas or by contraception and moving the animals to 'retirement homes'. Other species have not bred well enough to maintain their numbers - as we have seen with purple-faced langurs and Hamlyn's monkeys in recent years. Among the primates the species which seem to be rising in popularity include buffy-tufted capuchins (which is good as they are threatened) and white-faced sakis, which used to be thought too difficult to keep in zoos.
Alan