A)DPK is better than % based. % based means that you can simply never get it. Ragnarok Online showed this. DPK is already an improvement.
B)DPK has one simple problem: DPK is not personal. This means another player can get the item you farmed for, and you get nothing. This aspect alone is worse than the % system.
Solution: Keep DPK values as they were, but make them personal and persistent among multiple sessions.
Tada, everyone can farm things in peace. RETENTION is maintained, even more than now, because with the personal DPK, they WILL get the drop eventually FOR SURE.
Nothing is more frustrating than never getting it, but knowing you will eventually get it for sure will mean that everyone wins.