I don't think that there is an immediate threat to human existence. There are a large number of scarcely populated nations around the world, such as Russia and Brazil. Even China can sustain a lot more people than it is having right now. Also, the world population will stabilize sometime in 2050, and will start declining after that.
There are many scarcely populated ares in Russian and China
where people don't want to live. Yes those countries are huge but much of their land is tundra, desert, etc and not desirable.
I don't buy that the population is going to start declining any time soon. Life expectancy keeps going up and the classes that have more children per family seem to be growing faster than the classes that have 0-2 children per family. Medicine is advancing at an alarming rate and we are already having problems paying for medical care because people just aren't dying like they used to.
Creating a human presence off Earth does not necessarily mean and should not necessarily mean Mars, read up on O'Neil colonies.
Creating a human presence off Earth means having ... somewhere ... serious building capabilities, like the ability to build spaceships, motors and tanks.
That is a valid point though I am not sure if I have heard of O'Neil colonies. Can you post a link or two and/or a basic explanation of O'Neil colonies?