Well, I'd say the game already has all the widely known high profile zoo mammals. Otherwise though, absolutely agree especially with the whole "moving of the goalposts" thing. 3 to 4 years ago, the Tasmanian Devil, Wolverine, Walrus and Coati (to pick 4 more currently wanted mammals) would have been considered niche additions. Sure, every animal is probably someone's favorite, even way back then, but to consider them high profile additions way back at launch or first year would have been a big stretch.
It actually speaks a lot for the mammal roster that we're down to these animals as the most wanted missing mammals. Aside from a general shortage of New World Monkeys, basic zoo mammal representation is complete. And trying to say it's not, it's really kind of ridiculous to compare what's most wanted by taking someone like the Tassie or Coati and putting them on the same level of animals that were added early on like the Kangaroo or Penguin (not a mammal but the same idea is there).
If Frontier stopped releasing content at this point, I think many of us, myself included would be disappointed about certain animals not making it in. But I could not say that the zoo roster was lacking anything basic at least mammal wise.