I think that even if they did do away with F, VF, etc prefixes and just used numbers that someone somewhere would translate them to what they are on the usual F, VF etc scale (they'd probably have to translate the numbers for people themselves). People are simply very used to the established scale and with everything else that isn't slabbed being graded using said established scale it's silly to start grading things using just numbers. We need our F, VF, EF, UNC!