They're Not All That Wrong
It doesn't help one's case in arguing against the extreme position taken by the WWF to take an equally extreme position in the other direction. Then, neither side can really marshal the facts in its defense.
Yes, what they're aiming at is to impose global poverty. But they're quite right if we instead go on with business as usual, more species will go extinct as people intrude more into the few remaining untouched wildlife habitats.
There are basically two alternatives we have to the choice between a decent, prosperous life for all of humanity on the one hand, and the survival of the glories of wild Nature on the other. Two ways we could have both.
One is population control. If we decided not to have very many children, and our children decided not to have very many grandchildren, eventually, barring major advances in gerontology, there would be few enough of us that we could live comfortably without destroying the planet.
The other is nuclear power. That lets us produce the abundant energy our lifestyle depends on without impinging on the biosphere to any significant extent. We still probably would have to ration airplane travel, but that would not be a big deal if people could travel from London to New York in hours on high-speed electric trains.
You know, through the Channel Tunnel, and then on to the bridge across the Bering Strait...
No, I'm not kidding. We have options. We can build a meaningful future for humanity that doesn't mean ruin and destruction for the natural world.