To say a zoo is essential for any of the above is odd when there are plenty of alternatives such as conservation centres and animal sanctuarys.
Zoos offer facilities and services that you can't get in conservation centres and animal sanctuaries (e.g. breeding programs for endangered species, like the chronically stupid Giant Panda). They also offer much easier access to the animals, and keep them safe from poachers. I believe conservation centres and animal sanctuaries are essential too. We need a mixed approach to conservation, because there is no 'one size' that fits all.
As for education, you can argue video or streaming an animal acting naturally as it would with no people crowding round would be more educational, even if more pixelated.
You can argue that, but I don't find it convincing, and in any case there are plenty of zoos that provide facilities which prevent people 'crowding round.'