You're assuming life after the journey was guaranteed to be better, but not all colonists and immigrants happened to head to the world's future superpower.

Every decision is potentially committing descendants to the consequences of that choice (and to wit: life aboard a generation ship hardly need be a miserable or undesirable one, at the size of say, a large town and surrounding hinterland you have as much or more opportunity as anyone else at most times in history - I think generation ships force us to confront uncomfortable questions about what is the meaning of life on Earth which we try to sweep aside by deciding they're an impossible moral burden).