You say that as though it's a matter of fact
Think of it this way - we actually know how to travel great distances through space in theory, we just lack the ability to actually pull it off yet. Compare that to - for example - when we were learning to build a flying machine (not that long ago) and were fumbling around blind......machines that flap, getting up a head of steam and plunging off a cliff, etc, etc. Eventually we worked out that it was the shape of the wing that mattered. So in a way, we're in better shape to eventually traverse the vast distances of space than we were of building an airplane. We understand the science - we just have to wait for the technology to get there.
I think it's a pretty safe bet that we'll be flying off to other galaxies in hundreds of years from now.......assuming we're still around as a species. And when we do, we'll probably find a planet full of aliens that hang around on forums saying "pffft - aliens LOL you nutter"

But yes, it would still mean that back here on Earth we would only find out the success of the ships 100 years later, which means it could be 3-4 generations of humans between blast off and finding out if they succeeded. What if these people got forgotten about over those 100 years? Then would we think we had found another group of Humans in the galaxy?