If you allow for INSTANT POWERDOWN even with a tax then you effectively negate the protection that Hive gives to people that have their keys stolen. (A hacker doesn't care if they get 10% less of what they steel if it means getting it all right then)

If you allowed this Then you could easily make the appeal that all powerdowns could or should be instant if you wanted and have a tax or not if people wanted it. OR you could even do a scaling powerdown if you wanted from 0-20%

But i do remind that PART of the reason for a delay is this unique protection feature that Hive has that other blockchains do not.