Originally Posted by EWR764

Certainly possible, but generally aircraft are retired a limited number of hours before a heavy maintenance visit, which could easily cost $1m or more for an airplane that old. If N505UA is close to such an interval, United would have to make a cost/benefit decision to bring it back and, if necessary, perform required maintenance that will result in additional downtime. No doubt United network planners are running through utilization scenarios to see whether the loss of a 757 can be absorbed.