> suddenly we'll be in other planets.
Who is “we” in this case, how does it affect you and me? (The point being is that such long-term projects are beyond current society capabilities)
I don't see any scenario where a generation ship can plausibly be expected to colonize another planet.
While at first it might seem that Mars or any other planet is the solution to our environmental problem, the easiest solution to the problem would be fixing this planet. Colonizing Mars would take up so many resources that fixing Earth might be more cost-effective.
Teraformation and space conquest are fassinating topics, but "fixing our behavior to avoid to fuck up the only accessible viable environment we have" is far from deserving only a mere "might be more cost-effective".
Or we could, you know, just transfer our consciousness to machines that can easily survive Mars and other environments and start our colonization after declaring ourselves to be Homo-sapiens-machina.
Our current gas emissions would disagree.
Also, asteroids, gamma ray bursts, curiosity, sheer ambition, etc, etc.
The thing is: we don't need to convince all of mankind to colonize another planet. All it takes is a few people. Crazy rich Mr. Musk is one example. He may not be able to achieve it, but maybe all we need is a few other Musks in the next generations and suddenly we'll be in other planets.
The Fermi Paradox discussion is fascinating.