Typical situation. Derailleur goes into a wheel (for whatever reason) and rear derailleur cage is bent. If you were a customer would you rather have the derailleur replaced and know it is fixed, OR have a mechanic attempt to repair it by bending it back or replacing the cage and accept the problem may not be fixed the first time?
Yes, the replacement is more expensive, but is a sure fix. Replacing parts or manual adjustment may be cheaper, but introduces doubt into the repair and in all likelihood may work in the shop only to fail in use. Then a customer would return, upset the repair failed.
My stance is replace it and remove all doubt. Been asked as a mechanic to "make do" one time too often and want some outside input.
Repair or replace?