I don't think you have tbh. We're not as old as you think.
Experiencing Auckland's new precinct has somewhat inspired me re. the Islands expansion. If I hadn't seen and experienced it, I honestly don't think I would have that opinion. But it's really one of a kind, and I myself would 100% want to go back to experience it again. I'm sure the public would feel the same.
And it's not like the elephants would leave a MASSIVE gap. Obviously it would be, but I honestly doubt most of the public would switch to visiting Werribee just because they have Werribee. Even with the upcoming expansions, I'd still have Melbourne as a priority personally. They'd still have much more overall attractions than Werribee.
In saying that, Werribee's closing in. They have Gorillas, Lions, Hippos, Giraffe, Elephants, Cheetahs. Melbourne have Gorillas, Orangutans, Giraffe, Baboons, Seals, Lions, Snow Leopards, Tigers. As you can see, the Orangutans and Tigers are two of the main points of differences Melbourne would have over Werribee. Imo they need to take advantage of that by implementing the expansion.