That's actually not true as regards US Airways' business. While total demand is higher on BOS-LGA and LGA-DCA than on BOS-DCA, US Airways carries more passengers on BOS-DCA than it does on the other two routes. The average numbers for 2002Q2 were:
BOS-LGA: total daily pax=4531, US Airways share=32.09%=1454 pax.
DCA-LGA: total daily pax=4008, US Airways share=40.89%=1639 pax.
BOS-DCA: total daily pax=2587, US Airways share=71.80%=1857 pax.
Delta leads in market share on the first two routes, despite having slightly higher fares. Right now, shuttle traffic is terrible due to competition from the Acela Express and increased hassle at security checkpoints (as well as a depressed economy in general). When you consider that US runs roughly 15 daily departures between BOS and LGA daily, and that those passenger numbers are both directions combined, US only gets about 50 O&D passengers on each of those flights (there's some connecting traffic). That's not really enough to justify the A320 (or the 737-800 for Delta) anymore.
As far as most passengers can tell, if the interior looks and smells new, it's a new plane...