Uhm, the “goodwill” of fans has been bad on youtube,
Not to say YouTubers aren't people, but consistently the complaints
I've seen from popular YouTubers (kind of ad nauseum at this point) are from
builders who want more items and particularly new mechanics to design around. Many of them, like you alluded to, came to Planet Zoo from Planet Coaster and what brought them is primarily the building and design aspect of the game. A free animal would do... basically nothing for that crowd. It would get them through one video, maybe two, and then they'd be back to discussing how they want more unique habitat components, like aquariums and aviaries, because that kind of thing meets what
they want from the game and offers them new ways to make content.
They're allowed to want that, of course, and I also would love to see some aviary pieces! But I think it's a mistake to assume that a tiny selection of well-known and vocal players are an accurate sample of the community at large. They're part of the community, but far from the be-all and end-all of people who enjoy and support this game. There's plenty of people who enjoy building but aren't as motivated by it as people who are having to make interesting, fresh content as their job; likewise, there's plenty of people who are here for animals, and building is more or less an afterthought and a supplementary feature of the game, not the core draw to play.
As many have already said, DLC so far has been selling well and is overall quite decently received, with the major complaint for DLCs (prior to the Southeast Asia pack) across the board being "we want
more." I think that does say something about how people are feeling about the game, and personally I read it as positive.
tl;dr: YouTubers who build for a living/monetised hobby aren't actually the best sample for average player feelings re: the game, in my opinion, and they aren't the best barometer for goodwill towards the game, given that they have fairly specific agendas re: what makes good content for their channels. A free animal for the anniversary wouldn't really do much there, I don't think.