I don't disagree that he underestimated ISIS. That said, another 10-20 year occupation (which I agree would have been required) simply wasn't and isn't worth it. Many more than 3500 would die over that 10-20 year period, and the continued costs hurt our economy.
The failure to maintain what was won at a cost that isn't worth it is not nearly as big a blunder as the action that resulted in the power vacuum in the first place.