No, it's more of a No True Scotsman. People have written DSLs that are unreadable, but those DSLs weren't Well Designed(TM). So their failures don't count against DSLs, because only well designed DSLs count.

I think empirical evidence is that DSLs are easy to make not readable, especially as they evolve. Then again, that's true of almost everything, including assembly language programming, structured programming, object oriented programming, and functional programming (did I miss anything?).

I was talking about the well designed DSLs. This should include a decent syntax.