Yeah imagine all the big whales powering down and exiting in one day or even 3 days, witnesses will be affected immediately because all of the missing voting power. This will probably destabilise the blockchain if it's implemented.
A middle ground needs to be found
I was thinking more along the lines of if someone's account was hacked. Right now if someone's account is compromised their steem is still safe as long as it is powered up, giving you time to recover your account. However, if there is an option to instantly power it down, it means having your account compromised could mean loss of all funds, even if powered up.