I think you had it right when you started. Their concern was around, I think, pending applications. Previously you were required to have a registered mark before you could file using Madrid. There were also concerns around what you mentioned as a central attack, where if your international registration falls, that you lose those applications or registrations in the foreign countries. Those have now been worked out, and the U.S. is a member of the Madrid system.
On May 5th, 2014. See this statement in context.