While some of these visionaries have failed in their own lifetime, their contributions to society is incomparable. We have always relied on those with a vision of the future to guide us toward that future. All of the technology we have today was indeed envisioned by others decades ago. That vision guided us to the reality we have today. And today, there are two almost diametrically opposed visions as to what our future will be - outward or inward looking.
But the technical issues pale in comparison to the social ones. Not only will almost everything have to be lifted from the gravity well of Earth in order to establish civilization beyond the bounds of our atmosphere, but civilization itself will be fundamentally different from that on Earth.
And yet, it seems like we have been in this situation for decades now, perhaps longer.