The only account that can effectively power down to sell steem and not be penalized would be @steemit. What if I wanted to power down to continuously buy SBD but am not leaving the eco-system? I'm not saying that the idea isn't without merit, but I would rather see more incentive to POWER UP as opposed to disincentives for POWERING DOWN.
For instance, one could have discounts on promoted content if paid for in STEEM POWER.
A market for Steem tokens that become locked like with the @null account sounds like a solid idea to me.