I know that this quote exists, i just don't know who said it or the specific language. The quote goes something like this:
If we ever decided to send a manned spaceship (generational ship) to another planet, that spaceship would be be passed by a more advanced ship (one that goes faster) before it ever reached the planet, but if we don't send it then we'll never have the technology to send the second.
And the quote, i think, also said something about this happening two more times before they reached the planet.
Hoping someone can help me out with an author/speaker, and the source that i could cite.