At some point, it becomes throwing good resources, human and otherwise, at a failed venture. Mid course corrections are the duty of an executive, with a careful analysis of the costs and the benefits, and the impact on the Fruited Plain. I certainly hope that Bush's one last throw of the dice, wins the pot. That would be grand, and beneficial to most all on this planet. The alternatives are all ugly. The odds are that we are faced with a series of alternatives, all of which are ugly. Sometimes things just go bad. It is part of the human condition.
What if the 20,000 additional troops doesn't work, because the correct thing is 100,000? Does failing at 20,000 mean we are forbidden to put 100,000 in and fix it? Do we have to fail just because he's used up his "tries"?
This isn't a football game where there is 60 minutes and the game is over. This is a real life war where, if what we try this week doesn't work, we actually have to figure out what to do next week.
And for the forseeable future, whatever we choose will be better than the alternative of "pull out the troops and turn Iraq and it's oil over to Al Qaeda". Even if it's costing us a thousand troops a year, that's a fair price to pay for NOT having Saddam or Iraq giving WMD to our enemies, or having their oil revenue paying for suicide bombers in Israel.