<To the technologist, the answer to why is far easier: to get mankind off this rock.

If I weren't a space addict, I'm afraid I'd have a different opinion. It's important to find all the arguments for or against what we are interested in.

Mankind occupies a tiny fraction of planet Earth.
There is still plenty of place here; the oceans aren't occupied yet.
There is no planet, in our solar system, that can allow mankind to thrive like on our own Earth.
Even if there is a global warming, an asteroid collision, conditions could become unconfortable, but they would still not be as unconfortable as in space or on another planet. A civilization-killer asteroid would have to be large, and large means easy to spot in advance, so very likely, we can be prepared without even having to go up there , personally.
If the argument is about changing ourselves in order to adapt to space or other worlds, the same can apply to changing ourselves to adapt to our own changiing world.
All those bubble-cities on the Moon or on Mars might one day exist...on Earth.

The really dangerous conditions the Earth will have to expect, are so far away in the future (like oceans disappearing because of the Sun's increasing heat) that it wont involve mankind anyway.

Raoul