We don't have any such power. The exchanges remain powered up, and in possession of the stake requisite to effecting governance of Steem at their sole option - which they have already demonstrated willingness to do.
Wrong, thankfully. https://steemd.com/witnesses
You're saying this figure includes Steem on exchanges?
That figure is a Steem (the network software) statistic that tracks how many blocks were produced compared to how many witnesses were scheduled recently. It's there mostly to indicate forks, so a forked witness or group of witnesses will automatically stop producing if their software sees that the participation rate is say less than 25%. It's a setting in the software's configuration file.
Ok. Well, my guess was wrong. I see we have ~half the consensus witnesses, but I fail to see why the exchanges couldn't quickly change that. They remain powered up. Pretty sure that's why.