The problem with this analysis is that it assumes people will only do things that make economic sense, and only if it will benefit themselves or their descendants in a short time frame. In fact, we've done all sorts of useless, expensive things "because it's there," and we spend money on things like radiation shielding for nuclear waste that will last thousands of years.
Granted, space colonization would be much more expensive than any very long-term or symbolic project we do now, but it's not out of the question that future societies would be more inclined to do stuff like that than 21st century anglophones. If so, none of Stross's barriers are necessarily deal-breakers.
For example, I don't think Hitler would have spared any expense to seed another world with Aryans, regardless of whether it made any economic sense (since when do humans only do things that make sense?), and the 420 year time frame wouldn't seem like much to a man who thought he founded a 1000 year Reich.
Granted, space colonization would be much more expensive than any very long-term or symbolic project we do now, but it's not out of the question that future societies would be more inclined to do stuff like that than 21st century anglophones. If so, none of Stross's barriers are necessarily deal-breakers.
For example, I don't think Hitler would have spared any expense to seed another world with Aryans, regardless of whether it made any economic sense (since when do humans only do things that make sense?), and the 420 year time frame wouldn't seem like much to a man who thought he founded a 1000 year Reich.