This has been my basic take on it.
However, if you want a point of concern, I'll throw out some alternatives. I'm going to ignore generation ships, because those seem a trifle unlikely (too complex, rather cruel to the inhabitants), but let's imagine some embryo-laden carriers, freeze-dried aliens, and von Neumann probes.
Now, again, that's still a long way for very little gain. There's nothing physical you could bring back to your civilization that would be "worth" the trip, but what about some irrational civilizations? Religious zealots, or just a large enough portion of a technologically advanced society that says, "Screw it, let's hurl some madly self-replicating probes out into the spiral arm." Ninety-nine percent of their civilization might think that is a terrible idea (or pick your proportion), but there only needs to be a few to pull it off.
Why are we not seeing the von Neumann probes? I will admit that the other scenarios are less likely. But where are they? Even at a ten thousandth the speed of light they would have swarmed the galaxy, replicating down one spiral arm, into the core, and back out along the remaining arms.