Note that a declining number of species doesn't mean fewer living animals.
As the world globalizes, 10 isolated bird species occupying an ecological niche can be replaced by 1 species across the planet, without there being fewer birds total.
And millions of animal species occupying thousands of ecological niches are being replaced by 10 species across the planet: humans, the animals that humans eat, and the animals humans keep as pets. Even if the quantity of biomass is relatively constant, I think we can all agree that a planet with only humans and cows and chickens is a pretty bleak outcome.
I expect the endgame here is the whole planet is a managed park. We'll still keep Yellowstone and Serengeti looking as is, but there won't be any true wilderness.
There is nothing unsustainable about this, but I agree that it is quite dull.
As the world globalizes, 10 isolated bird species occupying an ecological niche can be replaced by 1 species across the planet, without there being fewer birds total.