I agree that the post-colonial order is dying, but I think you're overrating the influence of former colonial powers on that order. In the Middle East the post colonial order has been dominated by long-lasting dictators like Assad, Gaddafi, Hussein, etc, and by the royal families of the Arabian peninsula. The dictatorships are expiring, with unpredictable results that may or may not include the redrawing of borders. The monarchies seem more durable, for a variety of reasons, though how much more durable remains to be seen.
Post-colonial dictatorships will inevitably fall, and the process of political evolution that was interrupted by colonization will resume. he challenge for the US in all of this is to determine where US interests actually lie (to the extent that there are any) and to develop practical and achievable goals. I think commitment to the British and French is less an issue here than the traditional US confusion over interests and goals.
In parts of the Middle East the US does have a clear economic and strategic interest: oil. That doesn't necessarily translate into good decisions, but at least it produces a bit of clarity where goals are concerned. In other parts of the Middle East (e.g. Syria) and in most of Africa, the US has no compelling economic or strategic interest in place, which creates confusion over goals. When the US does act in these areas it tends to do so on vague "humanitarian" grounds, with limited commitment and sustainability, as will always be the case when there's no compelling national interest involved.
The US doesn't really need to understand the complexities; they just need to be able to determine where their own interests lie, if there are any interests at stake. As is always the case with a status quo power, the US tends to reflexively prefer the status quo, which is not necessarily an advantage. The US is also heavily driven by domestic politics, especially where no discrete national interests at stake, and is inclined to act, often hastily and without defined or practical goals, when a situation gets a lot of media attention. This confusion is to me more a problem than anything the British or French are doing.
Ultimately I'd say the best US policy will be the one the US adopted toward post cold war Latin America and SE Asia: back off and let them work it out for themselves. Solving other people's problems is not our responsibility and we generally do a piss-poor job of it, largely because we're never quite sure what we're doing there in the first place.
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