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.