I re-read your initial comment and apparently I misinterpreted what you wrote, so thanks for the high-quality answer despite this. (You didn't explicitly call out Dyn for not double-sourcing, but their customers, and only used it as an example for good practices in the Verisign example. Since you mentioned it lot, I gave it to much weight and read it as "Dyn failed because they didn't double-source", for which I see no evidence.)