That kind of thing is often said, but it doesn't make much sense to me.

Preserving the planet is valuable insofar as there are people to enjoy it. Without people to experience it, it wouldn't matter much. Without any sentient being to experience it, it would matter (in my view) zero.

Thus, creating new people to experience it is necessary, if preserving the planet is going to be worth it at all.

Of course, one can argue that there are too many people and we need to make less, but I think there are lots of lifestyle changes that need to be made before something so radical even makes sense. I.e., don't think people should make less kids so that the ones that do get made can live in a suburb and drive a car everyday. That's getting the priorities totally wrong.