I don't think the sales of the Fiesta are that bad within the segment...I think the whole segment of sub-compact cars is not selling well, as compared to the compact segment.
Subcompact Sales in November 2010
Nissan Versa 6,724
Honda Fit 4,180
Hyundai Accent: 4,052
Ford Fiesta: 3,473
Chevy Aveo: 3,262
Toyota Yaris: 2,154
Suzuki SX4: 1,140
Kia Rio: 966
Scion xD: 810
Mazda2: 462
....For comparison, the Cruze had sales of 8,066 and the old Focus had sales of 13,030 for the same month. And that was the first month of Cruze sales. What it boils down to is that Americans just dont like sub compact cars in general, a certain segment of the market does, but there isn't a widespread demand. The demand is much stronger in the compact segment.
I agree, when the 2012 Focus hits, moving a Fiesta will be a tough sell. Soooo much more car for so little extra cash.