There is a broad spectrum between "slavishly adopt nasty secret anti-consumer treaties strong armed by the US behind closed doors" and "invalidate US copyrights just because". Suppose a few large markets among other things decided they don't need to please the US on this as much as they have in past and try some true copyright reform. Say they have a system where copyright only lasts 25 years and can be renewed once if it is still profitable for the copyright holder. Under that system, those two old ladies wouldn't be able to terrorize people for singing "Happy Birthday" around a campfire. There are many such possibilities where many US copyrights would (eventually) no longer be valid in those jurisdictions. And if the US continues as it has been, this could start happening sooner than you think.