more people is better than fewer people. it means more of humanity, more discoveries, artwork, music, philosophy.
the more we spread out the more freedom we have to pursue our individual dreams and aspirations. (I believe individual freedom is inversely proportional to population density)
the more we spread out the less likely we are to become extinct in a single catastrophic event.
the instinct to explore discover and expand is intrinsically human. Implying humans should stay on earth is about as silly as thinking humans never should have left a small valley in Africa that could only support 100 people.
to stay is to die and become extinct. that might be preferable to some. and you are free to do so yourself, but please don't prevent those of us who have that unquenchable fire from pursuing that which will set our entire species free.