That's a terrible risk diversity argument. There might be thousands or even millions of human generations before the next asteroid big enough to destroy human civilization on Earth. Taking fifty years to say, first develop the alleged "Star Trek" utopia wouldn't significantly change the risk for us.
Instead, I think far more mundane risks are better to consider. For example, there are routinely large economic downturns every ten to twenty years. A larger economy due to a space-side presence would help to smooth out the effects of such recessions.Or if there's a large war or disaster, there would be more resources available to help mitigate the harm.
Obviously, they don't provide a "killer ap" for space development, but there really isn't one over the span of a few human lifetimes except the opportunity to be at places that are completely alien to us on Earth and do things that haven't ever been done before.