As this point in time, I have to ask: do we really need to do it? Space exploration, as romantic and exciting as it is, is luxury. Luxury we can't afford right now.
The billions needed for a manned mission to Mars could be well spent in other areas. Might be a tough thing to say, but it's the truth.
I also disagree with this.
Beyond the unsupported dictates you've used ('it's the truth'), finding a new niche for humanity to expand into, historically, always lead to increasing prosperity; on the other hand, social programs historically proved highly inefficient in combating poverty, etc.
Plus - overall, humanity is today richer than at any other time in history.
If successful, Musk&co will do more to ensure humanity's prosperity than all the social programs in existence put together.
PS - You obviously have no idea of the resources to be found in asteroids.