I guess that is possible, but I suspect it would not be to different from let's say flags on posts today. There are repercussions to acting with aggression.
Here is my point and I'm sure I'm not the only one who is thinking about this. There are many dead witnesses who still have heavy votes on them, as a matter of fact some of these witnesses have not been active for over a year, they don't run servers, they don't exist, there are ghosts from the mining days I'm told.
I speculate that they exist only to work as levers against certain witness positions, meaning that witnesses like @sircork and @ura-soul for example could purposely be pushed down in the list by keeping these ghost witnesses positions in zombie mode.
Why would I say such a crazy thing? Because some of the big votes that are given to these dead witnesses come from big accounts, people that know they are dead, super users that know better. This list of zombie voters include other witnesses.
I accept that there might be another explanation out there, I just can't think of one and I've been trying to wrap my head around this whole thing for a while now.
So why downvote witnesses?
It could help clean up the rankings by removing the zombies, hence make the playing field a little more fair. Would this eliminate closed door deals, corruption? Possibly not, no such thing as a perfect system, but maybe it would be enough as to allow people who are working towards the health of steem a better fighting chance at making a living doing so.
Just sharing my two cents for anyone who might not know about this.