Remember they went through a couple of variations of prestige loss/gain in the old gym system, so it's possible they may introduce increased motivation loss if a defender is beaten by a lower CP attacker - 100% speculation on my part and I get there would be some implementation issues with that since the defenders CP is constantly decreasing.
Bottom line, I'm not transferring any former prestigers until I need the space.
Pokemon used for prestiging
With prestiging a dead art, is there any point in keeping any of my old lower CP Pokemon?
Answers
I think depends on the moveset and IV of your lower cp pokemon. I still keep my several IV91+ with best attacking moveset not powering up. Will have more trial and error for gym battle and decide if they are worth powering up (personally I dont mind having several mon with same moveset as now maybe useful for raid battle - at least you hope to choose few machamps when encountering raid snorlax).
I keep the best of the best low CP mons just to save up on revives. Since the current gyms make their CP lower and lower, this means eventually you can beat said gym with a Pidgeot and save up on revives, maybe a potion or two to heal that Pidgeot. I used my 100IV Murkrow to finish off a gym that became very low on motivation.