They already do call for depopulation. While most of the world has decried the huge unjust deprivation of China's population to procreate ad infinitum, radical environmentalists have long hailed the policy as green, and in need of spreading.
The planet is a tool, and a resource. As it is also our primary residence, it should be kept pristine for our habitation. But it has no purpose beyond our sustinance, and without human life is meaningless (at least insofar as "meaning" is something humans provide.) People, please get a life.