The 'in a perfect world' line of argument is one which has been put forward frequently, and it really bugs me, for all manner of reasons...
1. It's indicative of the fact that some people in the zoo world aren't really zoo people - they work in zoos, but would really rather be doing something else to do with animals. (I really don't think this applies to tropeano at all).
2. If an animal can be kept 'happily' in captivity, we can justify keeping it in captivity. If it cannot, we cannot. End of story, regardless of its conservation value. As Pertinax suggests above, it is erroneous to think that all of Colchester's - or any zoo's - collection is vital for conservation. It isn't. But I don't think their meerkats, for example, are suffering by being in Essex rather than namibia - on the contrary, their lives are probably considerably less nasty, brutish and short than would be the case in the wild - so there is an excellent justification for keeping them for you and I (and others) to enjoy their place in our lives.
3. Seeing animals in the wild is wonderful, but it is a wholly different experience to seeing them in captivity. Neither is better or purer than the other.
In an ideal world there might not be so much fake rock-work at Colchester, nor the piped music, nor the garish signs - but the zoo would certainly be there, with exotic beasts to enrich the lives of those who pay to see them.