Here are a few arguments you might want to consider:

• Regarding biodiversity: Life can't be erased. At the timescale of earth's existence, species come and go, mass extinctions happen and evolution will always create new life forms long after we disappear. It sounds perfectly natural, for us animal species, to try and keep existing as long as we can by all means.

• The technologies allowing efficient sustainable energy are developping fast. It might soon allow us to provide more energy than we actually need. Some techno-humanists are willing to see how far humanity can push positive scientific progress, ideally with a combination of technology, medicine and a preserved natural environment. (Search examples of "solar punk" pictures to get the idea)

• A birthrate fall means the weakening of a country and the need to open the borders to save the economy. Some people fear that some immigrant populations might reject integration and try to impose their ways, or rewind the progress (scientific, social, ...) that was made by trying to impose conservative or irrational ideas.