Compared to anything that does not require brute force.
Imagine your account changed passwords every 10 minutes and you had a list of password to try every time you use it.
It would be safe and you would be using all those keys, I can't deny tou would be using, but it would be a huge waste of time.
Brute force is by definition a waste od resources and avoided in computer science whenever possible. PoS proves an alternative is possible so it is a waste to use PoW.