If we had wanted to "blow our wad," that might have been the thing to do.
Were we afraid of large casualties? Damn straight! Were we afraid that dire predictions by hand-wringing lib whiners' that we would become mired, like the Soviets, in an Afghan "quagmire" might come true? Yes we were. THAT'S WHY WE FOUGHT THE WAY WE DID.
Your recommendation might have been marginally acceptable (though I'm not inclined to admit even that much) if Afghanistan had been the beginning and END of the WOT, but it is not. It is essential that we maintain our resolve and carry this on, very possibly for much of the next generation. We can do that, but only by keeping casualties and other costs at a level acceptable to the American public.
Yes, using the North Alliance as a proxy force did cost some escaped rats, but it also won us credit with the Afghans, and with the American public. Giving the Afghans a major part in their own liberation will pay dividends in the long run. You would have shouldered them aside, blundered in with a huge force, incidently providing the irregular jihadi warriors with abundant and juicy targets in the extensive logistical support elements that would have been required, and generally made a huge mess of the whole thing. (IMHO)