There are bad zoos, but I favor the “more the better” angle since it reduces the chances of extinction or inbreeding and spreads the cost of supporting a breeding population out so that economic woes have less impact on the total number of specimens than if there were just a few well finced institutions. We’ve been lucky in recent history not to have truly great catastrophies strike but the fact is, they do happen, they are just as likely to strike in wealthy areas as poor, and when they do the species local to that area can go extinct overnight, even those contained in some leading zoo’s immaculate enclosures with all the amenities.
It’s better to have numerous individuals in their prime kept in both small and large groups in the event full scale breeding ever needs to be undertaken to replace or bulk up a species in the wild... and to have these dispersed all over the globe, rather than concentrated into just few large institutions that can be wiped out in a flood, eruption, war, or earthquake.
We could wake up one morning and the national zoo could be a smoking hole... while hundreds of “Johnny’s Roadside Animal Parks” out there, some better managed than others, are still full of healthy critters along with the usual collection of aged and crippled individuals. And with those, and the kids who enjoyed them and then went on into conservation careers, there’d be a reserve for that day things settle down.
Okay, maybe 20-30 in the USA. I don’t particularly care for taking healthy animals out of their natural habit and putting them in small zoos.