Regarding a payoff in 100 years, I think that's the GP's point. Private investors won't touch it because they have to think in the much shorter term. Even governments have historically not planned 100 years out. No one really has. And in this context, I shouldn't say 'planned' but rather 'executed'. Plenty of folks "sit on their ass and day dream". Few actually put anything into action with a view to things paying off well after they are dead.
That said, we (as a species) need to be thinking in these terms. Right now, the entirety of our species is tied up into the ongoing balanced existence on this one rock. Any number of cosmic events could wipe us out. For all we know, there's something heading toward us on an approach or in a manner that we can't detect and that will destroy all life on our planet.
While there have been other endeavors undertaken which weren't expected to reach fruition within the lifetime of the original actors, they were usually within a generation of completion. Speaking of social revolution, various technologies, etc. Full on independent existence of humans in a self sustaining manner on another planet is, I believe, going to be a multi-generational effort, something that if we start now in earnest, we won't see, our children won't see, our grandchildren won't see.
And that's the problem, as others have pointed out. We're, most of us, self-interested. That extends to our interest in our loved ones, those who are alive now or that we can imagine (kids or grandkids possibly). But few people are far-sighted enough and focused enough on the long haul to take these sorts of causes up. And the further down the line the payoffs are, the less tangible they become for those who would have to start acting now and the less likely any such effort is to get started.