Honestly a lot of the animal rights people (maybe I'm using 'a lot' with too much liberty) are fine with the animal dying under any circumstance, so long as they don't die in the one circumstance they don't agree with (ie slaughter, euthanasia, whatever). I've met some people while in my line of work that are totally down for animals to be 'set free' from zoos and absolutely acknowledge it would mean a death sentence. You usually hear 'At least they tasted freedom before dying' or 'better than dying in a cement cage' or something like that. So it's more they just don't like the idea of something rather than truly caring about that individual animal.
I also think the average animal rights person severely underestimates how stressful it is for an animal that has been under human care for the entirety of it's life, +/- been in the same environment (farm/zoo/whatever) for the entirety of its life, to suddenly be 'freed' and then find itself in a place that it does not know. People like to think that if they walked into a zoo and cut all the fencing, all of the animals would frolick around with joy but in reality they would either stay put right where they are, or get out and then panic and literally run themselves to death. That's maybe a less true for certain species but in general they are more comfortable in their familiar environments than people believe.