So you're suggesting that aliens would want to go on a one way trip into their future and never return to a home that they'd recognize? All on a CHANCE that we haven't nuked ourselves to oblivion during the journey?
Really, there are so many problems with this argument I don't know where to begin... Don't forget that they'd be detecting 50-year-old light from us before they begin their journey, by which time we could have nuked ourselves to hell and back. That's 100 years passing for them under the best of conditions.
If we assume that no matter can travel backwards through time, or faster than the speed of light, (which so far is 99.999999999999% proven true) then every time they undertake a multi-light-year journey they'd be saying goodbye forever to everything they'd ever known. (The first Ender's Game trilogy covered this in great depth.)
Now, add to this what I was saying before, that we'd likely be mere sulfur-sucking bacteria at the time we're first picked up by their telescopes, or the entire solar system here has long since turned into a gas nebula; or even condensed into a new solar system with new planets, and you'll begin to see my point. -Even their telescopes won't find us cause we wouldn't likely currently exist.
Time is BIG. -It's the hardest thing in the universe to overcome, hands down.
To make a point about time, I was throwing out ballpark numbers that might assume a craft went 99.99% the speed of light the whole time. The more extreme the speed the more time slows down for those in the ship. one-g spacecraft imagines a craft where the acceleration has to build and you can see how the results build exponentially. Round trips to nearby stars could be made and people would return to a somewhat familiar planet.
Yet due to the slow down of time, 15 years on this craft is enough to go from one side of the galaxy to the other! There are already humans willing to make a one-way trip to Mars, so I'm sure there would takers to be shot off into deep space. Robots and probes would be sent first though I'd imagine.
[ame=http://www.youtube.com/watch?v=bKjr5OFqhLo]YouTube - Stephen Hawking's Universe Fear The Aliens part 1 of 10[/ame]
[ame=http://www.youtube.com/watch?v=TFz4O7xifZ8]YouTube - Stephen Hawking's Universe Time Travel Starship part 2 of 10[/ame]