There's no way Treme is getting better ratings than The Wire.
I can understand passing on a Wire spin-off in the name of ratings, but a political drama would've done way better than Treme. In fact, with a white protagonist and a great writer like Simon, I'd say it would've done better than The Wire, ratings-wise. Doing it while The Wire was still on the air would have been risky, but it may've even boosted The Wire's ratings.