It seems clear that whack-a-mole banning is not terribly effective; since the main thing (I think) we want to disincentivize is unproductive mindkilled mass-downvoting, making that less effective (because likely to be undone after a while) might be worth the effort.

I doubt undoing the votes will disincentivize anyone. I suspect that mass-downvoting is either an expressive act or it's the stick in a conditioning exericise (disagree with me and watch your karma go down right now). In both cases undoing the votes many moons later is quite useless.