There are ecological, biological, sociological, and psychological problems that can never be solved to make this idea work. The physical problems of propulsion have capture your fancy, and perhaps they can be solved, but they are the easy ones.
Science fiction rarely is about scientists doing real science, in its slowness, its vagueness, the sort of tedious quality of getting out there and digging amongst rocks and then trying to convince people that what you're seeing justifies the conclusions you're making.
That's one of the ironies of our time: Right when we're on the edge of serious improvements in health care, we're also cooking the planet.
Apocalyptic thinking happens on the left as well as on the right, and in environmentalism, that's a terrible approach to take. Because it isn't true.
Statistically it was not greatly different than it had been for previous generations, but anecdotally it had become so prominent that every problem was noticed and remarked. The cognitive error called ease of representation thrust them into a space where every problem they witnessed convinced them they were in an unprecedented colapse. They were getting depressed.
Live for the future. A cosmic history read out of signs so subtle and mathematical that only the effort of a huge transtemporal group of powerful minds could ever have teased it out; but then those who came later could be given the whole story, with its unexplored edges there to take off into.