Ehhh I see where you're coming from but I don't think it's quite the same. Here on Earth is the default, and while each individual's opportunities are greatly affected by the circumstances of their birth and parentage, with effort and luck there's a fair chance to change one's stars.

Opting into an interstellar voyage is a significant reduction in opportunity for almost anyone.

And yes, the same could be said for a European colonist crossing the Atlantic to the Americas in the 16th century, and many of them did face starvation, exposure, etc, but it's different when you're largely committing yourself and your immediate family to those hardships, under the belief that the timeframe for "a better life" is the next generation. Committing intermediate generations is a different beast.

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).