Planets are over rated.
Your point about the dangers of having all of our eggs in one basket is a good one. But why would we even WANT baskets? Why not decentralize, distribute widely and acquire massive redundancy?
You also mentions asteroids that almost hit earth frequently....
So...if given a lemon (Near Earth Objects)...make lemonade
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We've been to the moon using 1950's technology. Today the tech is a little bit more advanced. Is it a stretch to think that we could catch a passing asteroid, hollow it out, put a spin on it (for gravity) insert an eco-system and live inside?
Kind of a like a cruise ship in space?
Why put our eggs on yet another planet..or two...
Why not have an armada of habitats?
Yeah, totally agree with you. But I think the process will be gradual. Start with a planet first and at the same time make plans for asteroids and stuff. I haven't read about any technology that will allow us to hollow out an asteroid, spin it and create a habitable environment there but if there is one thing that history has shown, technological advancements have always made the impossible possible, so, maybe one day.