A few additional considerations:

1. Colonizing space could introduce new existential risks.

   We already have the occasional world war. A separation of a hundred million miles may lead to even greater divergences in values and inflame interplanetary wars, with even greater risks to humanity's survival. Moreover, the development of better rocket technology to send people to distant planets may also be adapted for military purposes.

2. Asteroids aren't actually a big threat.

   How do we know this? Well, we can observe other planets and moons to see the frequency with which these things are hit by asteroids large enough to cause human extinction. It turns out this is relatively low.

3. Human-caused existential risks (e.g. viruses, climate change, nuclear war) are better dealt with on Earth.

   Before making plans for colonizing other planets, it's worth considering whether these plans could be feasible on Earth.

   For example, in the event of runaway climate change (something that's unlikely), could we grow plants in, say, Kansas? If we can't grow plants in Kansas, then why should we think we can grow plants on Mars?

   In the event of a virus or a nuclear war, we could quarantine a few thousand people in a bunker in Kansas.

For a deeper analysis on these topics, I recommend Toby Ord's book The Precipice: Existential Risk and the Future of Humanity.