So are you saying there's nothing that the V8 devs could have done to ease the transition?

To their credit, they warned us some months ago (April?) on the list that the InvocationCallback signature and Persistent would change. That was a new step for them - the first time i was warned about breakage before it happened. But without concrete change details and a reference build to work against, one can't even prepare for the change until v8 has released something. A flood and backlog of client breakage was inevitable the moment they decided to make the change.