You are making the assumption that they didn't intend this result. Or at least that they weren't aware that this would be the result. I find that quite dubious. As you say, it was obvious by inspection
OTOH, what would have been the result of disbanding the Iraqi army? You've created a bunch of people trained in violence suddenly out of work. I'm not convinced that it would have resulted in a better situation, though clearly it would be a different situation. And long term occupation would also have tremendous probabilities for disaster.
The real mistake was deciding to invade. After that I don't think there was a decent exit strategy...not if you are counting human cost. But this *must* have been obvious ahead of time, so clearly that wasn't their consideration. Who benefited? Who expected to benefit? How? It strikes me as a clearly political decision with only political gains.