If the rule doesn't change, then there needs to be a clearly defined rule about how to deal with "discard down to" and reactions that trigger during this process that may cause you to draw more. The problem with the current "one at a time" rule is that there's no obvious answer about what is correct in regards to the Militia/Tunnel/Watchtower/Catacombs/Sewers/Blessed Village/Sea's Gift interaction... which is a combination that comes up all the time, I'm sure.
Ok, so now it's just the Militia/Village Green interaction. Same issue as above, but is going to come up way more often. If discarding is one at a time, I have no idea whether it makes sense to say that you should have to keep discarding after you draw a card from Village Green or not. Either option seems equally likely. If instead discard is all at once, then Militia is a single atomic action that can never "keep going" after you've done it.
Interestingly, while MTG rules are insanely complex, the way they work with triggers is much simpler than Dominion... because things can't be interrupted by other things. If something triggers during the resolution of a spell, it just sits and waits until the spell is done before the trigger actually doing anything. If Dominion were like that, then you would always finish discarding for Militia before you gained a Gold for Tunnel, etc.