Even if we only consider Mythic clear, ML would still be better due to loot stacking on certain class. Even if all players were equal in skill and with the same class ML would still be better due to it. The increase from items you get just snowballs that much. Once tuning has been done we'll get the math on this.
Items can (and should) be assigned to classes/players that people know will be there to remove all wastage of loot.
PL will have some wastage of loot whether its bad rings/trinkets or armor pieces
PL raids can completely disregard armor classes (as in leave Mail users out of the raid) as that decreases the chances of trading and thus increases chances of wastage of loot.
PL Off-spec as lootspec on bosses to get weapons/trinkets for later trading (direct wastage of loot and yet most likely worth it for those pieces)
Unless PL yield more extra loot compared to ML than it currently does, it just won't surpass it in terms of effectiveness.