I would respectfully disagree with your assessment that finding a way to safely accelerate a big rock so it misses us is cheap and easy even in a relative sense

I would respectfully disagree with your assessment that finding a way to safely accelerate a big rock so it misses us is cheap and easy even in a relative sense. I don't believe it is easy in any sense of the word and it is fraught with potential dangers that would be impossible to accurately predict. For all of history humans have been going places and figuring out how to live there. Yes they've all been a different place on Earth but some of these places are so radically different from one another that the first sets of people died because there was no ability to even support them for a short time until they figured it out. We understand human biology and our needs well enough that these are problems that we could solve. It's also a scope that we can grasp. It is also not easy but most things worth doing are not easy. Missing a giant rock heading towards you until it is too late to do anything about is a very high probability. The only way to ensure survival is to put people in more than one place. If sustainability of our species is a thing we care about then investment in N+1 redundancy for humans is a requirement.