The DC-9's were actually gone *before* 9/11/01 -- their last revenue flights for US were August 18, 2001. The MD
-80's had been scheduled (as of mid-2001) to be retired by the end of 2002; they last flew in March or early April 2002, meaning that (1) their retirement pre-dated the US Airways Group bankruptcy and (2) the events of 9/11 and the poor economy only accelerated the retirement of the MD
-80's by several months.
All of US' 737-200 fleet (42 airframes) had been flying as Metrojet by September 2001; the airline had already proposed (as of August 2001, when "Plan B" was announced after the failure of the UAL-U merger) reducing the Metrojet fleet by a bit over 40% absent further concessions by employees. And in any case, by the time US ended Metrojet service in December 2001, the average age of that fleet was nearly 20 years -- the aircraft were nearly completely depreciated. This was also pre-bankruptcy.
Moreover, the F100's were also retired during Spring, 2002; again, this was several months before the company's Chapter 11 filing. This was probably the only fleet whose retirement was significantly accelerated by the company's financial troubles; the F100's were, on average, about 12 years old at the time. Chapter 11 was helpful in that the company was able to reject the mortgages/leases on this fleet (most of the F100's were owned).
Additionally, US had already reached an agreement with Airbus (in August 2001) to defer its 2003 narrowbody deliveries to 2005 through 2009. In late 2001, US deferred most of its 2002 Airbus deliveries, agreeing to take only nine A321's in 2002.
Interestingly, the company had already planned (as part of "Plan B") to reconfigure its 757-200's with fewer F seats for Florida service and to deploy its A321's for transcons. The plan after 9/11 but before bankruptcy was for the fleet to number 315 aircraft (down from 420 in July 2001). While most of the aircraft rejected by US were in the 757/767 and 737-300/400 fleets, US also rejected about 11 A320-family airliners in bankruptcy as well to reach their current fleet count of 279 (though a few of these are in storage due to the company's attempt to outsource Airbus S-checks in violation of its contracts with its mechanics). Also, the "Midatlantic Airways" plan was part of Plan B, but those aircraft were called "mainline regional jets" at the time.
While US Airways has significantly reduced capacity (by over 30%) since September 11, it is important to note that they had increased capacity by nearly 25% between 1996 and 2001.