"but I can't think of anything polite to say."

Then say something unpolite. It is quite possible there is something I have missed about the concept of singleton. I am taking the definition from Nick Bostrom's page.

Imagine singleton "taking over" the Mars. Every time intelligent decision is to be made, information has to round-trip with Earth - 6 minutes minimum.

In this case, any less intelligent system, but faster and local, is easily able to prevent such singleton's expansion.

IMO, the same things happens everywhere, just times are much less than 6 minutes.

To sum it up, light speed limit says two things: "smaller is faster" and "local is more capable of reacting".