Using things that weren't designed as security features as if they were security features is pretty much the least secure thing you can do. This applies both to keeping Steem Power in order to keep from being hacked and making long power-down times to discourage exchanges from powering up.
Powering up was not made for these purposes and there's no reason to believe it will behave well for these purposes just because it apparently has for you so far. This is like using a ziptie to lock your front door when keyed locks are readily available. Sure, nobody's come along and cut it yet.