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I don't believe that will amount to a very big problem. People will have fewer kids when they have more time and wealth on their hands. Besides, we might go to the moon/mars soon anyway.

There is plenty of room in the sky and on/in the oceans and great innovations might make those places inhabitable...

The moon/mars will probably be only for the super rich I think

Let us hope not, but yes we need to do something to avoid that scenario because it is a very realistic possibility.