It was your example, not mine.I'm not going to entertain that first point, because we both know it's ridiculous and I'm already tired of having my position caricatured. DCs are for discreet actions, not geopolitical goals.
What are the discreet actions required to achieve a geopolitical goal? As far as I can see, whatever the GM says they are.
In most versions of action resolution along the lines you are advocating, the application of swimming, boating etc skill would be related to the width of the river (whether that requires multiple checks - 1 per X distance - or sets a DC - difficulty Y to swim N feet rather than L feet). Who sets the width of the river? As far as I can see, the GM.The DC to cross is a river, in the kind of skill system I'm proposing, will be resolved by the appropriate swimming, levitating, flying, rope tying, boating, long jumping, or whatever other rules the PCs bring to bear against the challenge, no doubt informed by the speed of the current, presence of monsters, visibility and anything else going on.
Unlike a skill challenge, the number of times they roll will vary wildly, based on which of those approaches they took, the appropriate speeds of the methods of transport available to them as a result of those choices, and in some cases will be trivial, as perhaps someone actually bought a folding boat. Honestly, the only difference between the sort of system I'm advocating and a skill challenge model, is that DCs are intrinsic to the tasks being attempted and the effectiveness of any given check is specified by the action that allowed the check in the first place, instead of attached to a timer on the number of checks or number of successful checks.
Who decides whether or not a log is being washed down the river, and impedes the swimmer? Or is a boon? The GM.
The difference between your approach, and a skill challenge, is that on your approach the GM makes an ad hoc decision (or decisions) that determine what, and how many, checks are required to actually resolve the scene. The main point of a skill challenge framework is to make this transparent. Does a log get washed down the river? If so, does it impede or harm? The GM narrates this as part of the consequences of a successful or failed check. The authoring of the fiction by the GM occurs within the mechanical resolution framework; it doesn't create it.
If you want to see examples of tactical gameplay in a skill challenge (in the sense of making decisions about skill use, power use, etc), the example that @Campbell linked to has plenty.