The number seven spot, imo, is one for an all-rounder. Birt is not an all-rounder, so he is discounted. In 84 ODIs, Hopes averages less than Smith does with the bat, and almost 10 more runs with the ball. He's been given a big shot, and not grabbed it. He is, as I've expressed elsewhere, a very good State Cricketer, but not an International one, imo. Christian is no doubt, a very talented batsman, but, imo, a distinctly average bowler. His OD bowling average is 34, and more importantly, his economy is over 6. I also see limited potential for his bowling to improve.
I would disagree with you that Smith has done "nothing much of note for the Blues in OD cricket". An average of 35 with a S/R if 90 is very acceptable to be a numeber seven batsman for Australia. And while his OD bowling average of 34 is not fantastic, he has proven himself to be an effective bowler in domestic 20/20, and also in ODIs and T20 Internationals so far. Add to this that he seems to be developing with the ball in every game, and that I see lots of potential with him at age 21, I think he's the man for the job. You yourself have said he should be bowling his full 10.