You keep bringing up that zoos cause harm. This seems to be a deal-breaker for you. It seems your view is that if it's possible to achieve some amount of your goal without causing harm, you should do that, even if causing a small amount of harm would enable you much greater success in whatever it is you're after. In my view, it's acceptable to cause a small amount of harm, if you get significantly greater good from doing so. Of course the details matter, and I don't believe either of us would argue our position in every scenario, but in this case I find the manageable harm caused by zoos to be worth the increased interest in wildlife conservation.