The analogy between Afghanistan and the Oregon Country really doesn’t hold up. To amplify what OxyMoron already said, the dispute over the O.C. was a dispute over what was regarded as “empty” territory which the various players (mainly the British/Canadians and the Americans) wanted to expand into and settle. (Of course it wasn’t really empty, but what happened to the Indians in North American could spawn many threads all on its own, and at any rate, they weren’t party to the negotiations.) Afghanistan isn’t “virgin land” which various countries want to settle; it’s a country already inhabited by various peoples, who’ve lived there for centuries. And, if you look at the map, the lines between the Uzbeks and the Tajiks and the Pashtuns and so on aren’t easy to draw. (Remember too that that map is an oversimplification–even the solidly-colored areas aren’t 100% this or that group; there are people who are Tajik on the mom’s side and Uzbek on their dad’s, isolated villages of Group A in the territory of Group B, remote valleys of this group in that group’s turf, and big cities where members of many of the major ethnic groups rub elbows. There are also many fissures and subdivisions within the major groups, especially the Pashtuns. There’s also the factor that major cities like Herat and Mazar-e-Sharif, with all sorts of cultural baggage and strategic significance with regards to transportation lines and so on, are of very doubtful ethnic affiliation, and would probably be claimed by multiple sides.)
Certainly I think Afghanistan could break up along ethnic or semi-ethnic lines. I just don’t see how it could possibly be described as a good thing. It would almost inevitably lead to a bloody, messy war over the numerous boundary disputes and territorial conflicts that would spring up, and if you start trying to divvy up the country among Greater Uzbekistan and Greater Pakistan or Greater Pashtunistan* and so on, you’d export Afghanistan’s existing bloody wars to, at a minium, Uzbekistan, Tajikistan, Turkmenistan, Iran (to whom you’ve awarded the Shi’ite Hazaras), and Pakistan. India and China have no direct plausible territorial claims in Afghanistan, but that list of players includes enough “enemies of my enemy” that one or both could easily be drawn in. May I remind you that Pakistan has nuclear weapons? (And so do India and China, of course.) And at least Iran has definitely sought nukes as well as other advanced weapons (chemical weapons and ballistic missiles). Russia still regards the Central Asian Republics as being in its sphere of influence, and the U.S. has clearly taken a renewed interest in the region, not to mention that as a superpower, the U.S. has interests everywhere.
I’m not saying the partition of Afghanistan would inevitably lead to World War III, but it would almost certainly be a big huge international headache, not to mention a major humanitarian tragedy.
*As JRDelirious pointed out, amalgamating the Pashtuns into one country could be very destabilizing to Pakistan, where the Pashtuns form a large and somewhat restive minority group. Faced with a partition of Afghanistan, the Pashtuns could easily demand a united Pashtun homeland on both sides of the current Afghan-Pakistani border–which I don’t think has ever really been fully accepted by the Pashtuns/Afghanistan–rather than remaining only a somewhat more numerous minority ethnic group within Pakistan.