Whatever the limit is, there is a limit. Because there is a limit to the earth. Whether that limit of sustainability is 2 or 50 billion is something we haven't figured out yet but that there is a limit seems to be something you can't around.
Earth can only sustain so many people, and since we do not currently have a viable way to get off this earth and/or to terraform other planets (hell, we can't even agree on a common plug to use for household appliances) it looks like it will remain that way for a while.