Interesting proposal. I agree with you the algorithms could include rewards for users not powering down. For posts with 50/50 as well as 100% SP. Larger incentives to the 100%. Maybe: x to 50/50 posts and 2x to 100 posts. I state x, since x to me is not necessary 10%. It could even have a higher value in my opinion. Or some kind of formula including the number of power downs and/or relative and/or absolute value of power downs; The more power downs and/or value done in the past, the lower x will be (maybe it is even allowed to become negative). This will not prevent the real power downers to stop, but will keep the people who did not power down in the past, in power up position and therefore connected with the community. The people already in power down status (or done in the past) do want to leave, or at least something is in their mind that they do not want to be around for long anymore, so let them leave, clear up the community and build Steemit with the people having only positive in mind when thinking of Steemit.