It's not about what if we make this planet uninhabitable. There are lots of possible extinction level events which are not our doing. Humanity need more planets because of them.
Discussion
I think we have already survived extinction level events. I'm on team human and team earth. I don't want to waste resources to achieve a theoretical reality when we have proven resources right here.
For example, which one?
Do we have somethink which can protect earth from interstet asteroid like ʻOumuamua?