Re: " You build underground. "
No, you're actually an idiot. There's lots of stuff that could happen to the Earth that would wipe out any underground colonies. Especially if they didn't use a closed system. Plus, Earth is tectonically active; those colonies have extra risks there that they simply don't on Ceres or Vesta. (I never said the moon. The moon is a ridiculous place to colonize.)
Survivability of the species is about not having all your eggs in one basket. That means as many colonies as possible. Earth will eventually die. Period. Nothing we can do will stop that. It will be a lifeless ball of rock a billion years before the sun consumes it. Indeed, according to our best estimates there is less than a billion years left to this planet's ecosphere, probably less than half a billion during which it can support sentient life on the surface.
There is no good reason to stay on Earth except sentimentality. Earth is a great big gravity well where most of the really useful elements for high technology sank to the core long ago. Other than offering a magnetic feild and a trapped Oxygen/Nitrogen atmosphere it doesn't offer a hell of a lot we can't get elsewhere, and it has it's own problems to overcome.
What we need is to have colonies in small gravity wells. Ones where the cost of leaving the gravity well is negligible. We need colonies that can access resources like platinum group metals which make various flavors of high technology much easier. We need colonies that are not only self sufficient, they have enough resources to build colonies of their own.
You make the ridiculous statement that Earth having colony worlds decreases the chances of the human race surviving. You don't explain how that is possible. You just assert.
Would people on the colony world have increased risk compared to Earth? Yes. At first. Eventually, however, they'd adapt, the colony would grow and it would be as safe as Earth. Safer, actually, given that Earth seems to be filled with 7 billion humans all intent on wiping eachother out, while a colony would not only be a smaller and more homogenous population, they'd be focused on survival, not conquest.
And that - right there - is the biggest reason to leave Earth. Even if you have some sort of religious belief which prevents you from understanding that things like metor strikes can and will wipe out Earth-bound humans, the sad truth of it is that we will probably wipe ourselves out on this planet before long.
Humanity must spread to the stars in order to outrun it's own worst nature. It's as simple as that.
Adding colony worlds doesn't reduce the possibility of those on Earth surviving. It does make Earth irrelevant to humanity's survival in the long term.
The fact that you have such a fantastically poor understanding of science that you A) think we should live on the surface of a planet in a big gravity well as a colony world and B) think that a colony is particularly hard (as opposed to merely outrageously expensive) means you shouldn't be allowed to have this conversation at all.
We know how to survive in space. The #1 problem with space colonization isn't survival. It's that getting the materials needed to survive requires hauling them out of this accursed gravity well. Fortunately, that isn't a problem, long term.
We can send robots to Vesta and Ceres to refine the elements required for survival, construct structures, and prepare the way for colonists. We can - with enough money - assemble a ship that either has a massive fission-based power source which could generate a magnetic shield, or enough lead shielding to protect colonists on the journey.
That is all that we need. Once on Vesta or Ceres, with an army of mining robots at their command, the colonists will be able to create new ships and new colonies for a fraction the cost that could be accomplished on Earth. They will never want for space to expand, never have to murder eachother over ideology. If they want a place to practice their own vision of how things should be, they can just pack up and go. The entire universe will be waiting for them to do so.
Earth is a cage, not a lifeboat. This big, fat gravity well is a prison. The goal is not - and never should be - to create new Earths. It is to move beyond the need for such an incubator, and to explore the stars without the requirement for one large ball of rock filled with billions of us that can't get along.
All we need is that first little push. Not the moon, or Mars...but to new resources within easy reach and whose acquisition won't trap us for millennia. Earth is just one planet. It's not relevant in the grand scheme of things. Try not to get too attached.
We don't want to go to another planet because it will be safer for the individual than being here. The individual doesn't matter any more than the planet does. We want to make colonies both because we want to explore and because those on a colony world are safe when something does eventually happen to Earth.
Are the colony worlds more likely to experience catastrophic problems than Earth? Yes. But enough of them ensures humanity's survival. Whereas staying on Earth alone ensures humanity's demise. Eventually, all planets - and all colonies - will die. Every single person, no matter where they are, will die. But our species might survive, if we spread far enough - and fast enough - to outrun not only nature's worst tantrums...but ourselves as well.