Not sure if you missed the point of putting the existing plate onto retention.You wouldn't need to retain the plate, the DVLA (to the best of my knowledge) have never issues an older plate to another car unless it has significant value.
Even after 5 years I put my private plate on retention and got the old original plate back on the car before I sold it.
The idea is by doing so the DVLA have to issue a replacement registration number. This is useful if you don’t have/don’t want a cherished registration mark to assign.