I think this USO Series will die a natural death within the next one or two years as
1. The US is losing out on tourneys. Recent tourneys that got eliminated were LA for Men and Carlsbed for Women. You need to have tourneys in US to have a series.
2. Apart from dwindling tourneys, there is also not unity. Washington DC going on its own way and not joining the USO Sereis (due to appearance fee issues0.
3. The extension of the grass court season cut into the hard court warm-up portion of the season and that is another hindrance.
Let us face it. Currently, there are only 4 tourneys in the USO Series. Two of them (Montreal and Cincy) do not need any promotion as they are mandatory ATP 1000 events and all top guns will be there. One of them (Winston-Salem) is in the week before USO and no amount of promotion would really make top players go there and participate. So, basically the only real promotion that happens is for the single tournament in Atlanta. That to me is unsustainable situation.
It is exactly the same situation for WTA. Cincy and Toronto are premier 5 events and therefore do not need much promotion. Connecticut is in the week before USO and promotion would not help it. The only tourney that gains by promotion is Stanford. That is certainly unsustainable situation.