We've been here before isn't the same thing as 'this is fine'. The earth has had several extinction-level events in it's history - it wouldn't be 'fine' to have one again.
Well, the earth would be fine. The earth would carry on without us and it wouldn't be long until there were no traces of our incredibly short term existence. I've always been amused by "Save the Earth" campaigns. The proper phrase is "Save the Humans."