Extra-terrestial colonization appears to me to be one of the few avenues of human civilizational expansion that is positive sum for all lifeforms. We could grow our food in what are now lifeless surfaces, thus reducing the stress we impose on natural habitats, while massively expanding how much we produce.
Because Earth-based civilization relies on the commons, and it's hard to manage the commons, due to the vast number of competing claims on them. i.e. the tragedy of the commons.
Extra-terrestial colonization would be completely different, with entirely self-contained environments owned by defined parties who are incentivized to effectively manage them.
The other point is that extra-terrestial colonization could give us access to vastly more resources, that can be harnessed without putting any strain on natural environments. It could even be possible to return all Earth-based agricultural lands to their natural state, and grow all the food we need in urban vertical farms that receive the energy they need for plant growth from extra-terrestial sources like space-based solar panels.
