I think the premise of someone converting a lot of HBD being an "attack" is kind of wrong. It assumes that existing stakeholders don't value HIVE and have abandoned holding it to an extent that has caused the price to crash. In that case, maybe it's fair they just pass the baton.
The way I see it, if someone is willing to hold a lot of HBD while the price of HIVE is collapsing (risking losses, potentially total loss, with probably limited upside), and then convert their HBD, power it up, wait out the voting delay, and then vote, I think they earned their votes. Way back in the early Steem days, @abit did this, and he certainly earned his votes.
Someone trying to buy a lot of HBD quickly to pull this off, as opposed to being a long-term holder, would drive up the price of HBD, just as someone trying to buy a lot of HIVE would, maybe more so. We've seen how this plays out.
Someone buying HBD over time while others were dumping it could instead be buying HIVE while others were dumping it (i.e. able to accumulate a lot without driving the price up). Don't see much difference.