Now that you bring this, the thing is that at its current state it could be argued that the game isn't really the greatest zoo game, considering the year each game was released.
Planet Zoo is a wonderful game, no doubt, and the degree of freedom it gives you is unparalleled. It's by far the game that allows to build the most realistic zoos, not only in the building side but also on other areas like quarantines or veterinarians. The models of the animals are great (even the derpiest base game bears), the best we ever had in a zoo sim. In the last DLCs they are even the best animals I've ever seen in any videogame. But isn't that something that should be expected? I mean, the leap of quality of the animal models is as big between PZ (2019) and ZT2 (2004) as it was between ZT2 (2004) and ZT1 (2001).
At the end of the day, in a zoo sim game the animals are (at least) as important as the building and management systems. And the animal roster of PZ is still behind that of any of the ZTs. We had dolphins, sea turtles, sharks and other fish in ZT2, which is something that doesn't feel like it could arrive to PZ. And then ZT1 not only had all sort of marine animals (including invertebrates), but they also had bird houses and bug houses (with no creative freedom at all, but those graphics weren't suited for small animals). PZ has already surpassed the number of species any ZT had, but diversity is as important as quantity. Is it really better having 4 ungulates, 2 dogs and 2 cats (total 8 species) instead of 1 ungulate, 1 dog, 1 cat, 1 cetacean and 1 shark (total 5 species) and 1 uncustomizable bird house?
That's one of the big reasons I think it's important for PZ going for longer support and diversify the animal roster and mechanics. As it is right now, most of what makes Planet Zoo a better zoo sim is just inheritance from Planet Coaster and Frontier's staple building system. But its low-diversity species roster hampers the creative freedom that could be achieved with that great building system. The game calls itself Planet Zoo but right now it fails to reflect the diversity of zoos around the planet, it's more like Province Mamalian Park [forced laughter].
TLDR: As long as PZ keeps without fish

or birds

, it will fail to surpass what Zoo Tycoon meant to the zoo games community.
(Interestingly, both ZT went for marine animals expansions but with PZ and the WEs it looks birds are by far the most safe bet)
Now, a quick meme to make lighter my wall text: