Collapse is already happening for way too many, it will accelerate when no one on either coast can get property insurance due to rising sea levels, when the inland aquifers are finally sucked dry and agricultural irrigation ends, and when no more able-bodied children are available as cannon fodder for the empire. Probably the last straw will be when the never-ended civil war goes hot once again. We just need to hope they don't use nukes on each other this time.