That is what you are saying about leaving the Tajiks and the other ethnic groups in charge as opposed to a Pashtun.
While no nation wide census has been done since 1972, it is believe the Pashtuns are about 42-50% of the total population.
http://en.wikipedia.org/wiki/Demography_of_AfghanistanIf we do NOT consider the Pashtun, you are disfranchising up to half the population. That by itself would lead to Civil War, for the Pashtun will want to have a say in the Government.
A bigger problems is the Tribes in Afghanistan are more "like Jelly-fish" for trees in a forest. From US Army Intelligence report on the tribes of Afghanistan:
http://publicintelligence.net/u-s-army-human-terrain-system-afghanistan-pashtun-tribal-analysis/Basically the Pashtuns are not a Tribe in the sense of Iraq or other Middle Eastern Countries, but more like Native American Tribes, people who spoke the same langauge, traded with each other, intermarried, but at the same time would break up and form a new living arrangement if that is what a group wants to do and the rest of the tribe was to weak to stop them (and that is most of the time).
While the concept of tribe is weak, it still has some value to the members of that tribe and as such we have to deal with it. One problem is the problems of marginalization of the Pashtuns:
"http://www.rferl.org/content/feature/2250550.html
In simple terms, a Pastuns had to be picked, but the problem is we picked a western educated one, that was hopelessly corrupt. Thus I agree with you that Karzai has to go, but the problem then is who to replace him with? It will have to be another Pashtun, but Karzai has made sure no fellow Pashtun has stood out (other then one of his corrupt relatives, who we have to avoid). Karzai did this to force us to keep him as out best hope. We have to make the decision to replace Karzai, even if that means setting up the Taliban as the rulers of Afghanistan.