As far as I know, there's no in-game way of knowing if an attack is magical or not other than using your cooldowns and see if any mitigation has applied or not. For that reason alone, I'd like to just see DM work on physical damage too. If I have to go to sites outside of the game or look at guides on a fight-by-fight basis to know what attacks are considered magical, it's probably best to just change the ability and be done with it, especially since as you have pointed out that PLDs can just block magic now.