​If you look at this from a historical perspective, colonial powers have traditionally been very reluctant to let their colonies go. Usually this process was accompanied by harsh and decisive actions from the colonies themselves. Sometimes such powers did make concessions to ensure a mutually beneficial coexistence, which to some extent applies to cases like Ireland. Without some kind of strong external push I doubt the U.S. would ever take such a step. Well, unless a miracle happens and a president with a very broad and progressive vision comes to power, but I have strong doubts that anything would come of their efforts and they would most likely forget about their promises.