I won't even get into the technological benefits we enjoy due to their initial R&D for manned flight. You've directly benefited from man being in space, believe me.
I certainly didn’t intend to contest the historic benefits; I LIKE my microwave (which could as easily have been designed for nuclear submarines as spacecraft). But my point was that the manned flight R&D is pretty much done to death. We’ve been at it since the 1960’s and it doesn’t appear to have changed fundamentally. Look at all the work we got out of the Mars rovers and orbiters, and then think about the costs of just getting people there alive, let alone supporting them and returning them. We can advance knowledge faster going mechanized. It may be heresy, but I think Astronauts are expensive albatrosses given current propulsion technology. Think of the Predator drones: we can kill the Taliban just as dead without building in anything more than what’s required to get the warhead on their forehead.
I also disagree that colonization would be a panacea for overpopulation’s ills. We either learn to keep population within sustainable levels, or nature (and human nature) will do it for us, no matter where we are. Yes—with appropriate propulsion—Ancient man eventually found new lands. He then bred to the point that people are talking about space colonization as a way out of our resource difficulties. The more things change...
So, invent a propulsion technology that creates a leap similar to that between casting oneself adrift and harnessing the wind and I’ll be back in the manned flight camp post haste (because it is undeniably cool). But you’d serve humanity better by solving the fundamental problem of man being prone to outbreed his environment.