The instant power down concept, regardless of the cost/burn, destroys the security feature altogether. A hacker won't care about burning 5% or 20% or even 50%. Anyone who is hacked will lose all their funds instantly.

Also, proposing that people invest without powering up is ignoring the cumulative cost of Steem's high inflation. If you are day trading you don't care about inflation, but few serious longer-term investors can (or will) really accept an extra 8% per year for the "privilege" of investing in Steem, especially over a longer term holding period. That would be like paying an 8%/year management fee, which is almost unheard of.