I enjoyed this story immensely, for many of the reasons already stated in this thread.
In thinking about the ending, I think the author was wise to end it where he did. The implications in the story are clear enough, but getting there would be another thing all together. So the watchers found a reed boat, learned about it, made more, and were able to take a village by surprise. So they were able to sew by merely observing some clothes. All very impressive, and fascinating, and, well, wow. But still, even with a space ship and some supplies, spreading throughout the galaxy would be another thing all together. [Unless, perhaps, the good doctor took the Death Star with him on his expedition].
I just don't see it happening. And the alternate endings I have imagined so far just don't work as well as the actual ending of the story.
However, I fully concede that just because I don't see it, doesn't mean it can't be done. And if this were the beginning of a novel, I'd love to read the rest. But as a short story, I think the ending was right.