I don't see how this meshes with the facts of the matter. The line of thinking that leads you to the conclusion "Apple will surely open-source Swift" could just

I don't see how this meshes with the facts of the matter. The line of thinking that leads you to the conclusion "Apple will surely open-source Swift" could just as easily lead one to the conclusion "Apple has surely already promised to open-source Swift," which is obviously not correct. The fact of the matter is that they haven't and haven't expressed any interest whatsoever in doing so. The fact that it has no downsides for them and that keeping it closed will alienate people has clearly not been a compelling reason to even talk about open-sourcing Swift up to this point. So the question is, what would cause it to become compelling to Apple in the future? I don't think it's impossible that Apple would open-source it, but I don't think the evidence clearly points in that direction.