First of all, if we are identifying this flight as a "US Airways" flight, then yes, I agree that the flight number should be retired. Just as American Airlines has permanently retired flight numbers such as 191 (ORD, 1979), 11 (NYC, 2001) and 77 (DC, 2001).
Technically the flight is still an "America West Airlines" flight number and that is probably where the oversight lies. Over the years our mainline flights in the Phoenix-El Paso market have always been operated as flights 421, 423, 425, 427, etc. America West Airlines, thank God, has never had to permanently retire a flight number in their history as a result of a tragedy. However as we begin merging operations, I do believe that these factors do need to be recognized and respected.
I began my airline career with American Airlines and would be personally taken aback if AA were operating a Flight 191 (still the worst single aircraft accident in US history).
I seriously believe that this is an oversight that can be easily corrected as long as the right people are informed (who decides and can change flight numbers anyways?). With this merger we have taken on the history of US Airways...both good and bad...and should be sensitive to this history.