The distinction to make is that when there's above 90% of Steem in Steem Power (the actual number is a little lower) then the 9/10 Steem awarded to SP holders will be shared with more people so there will be an effective negative rate of return holding Steem Power.

For simplicity, lets say we have 95% vested in SP, the 9/10 reward will be relatively smaller as there's an extra 5% of holders to pay.

If we have 85% in vested SP then those SP holders get a larger proportion of the 9/10 reward.

On top of that SP holders are essentially taxed by the 1/10 rewarded for content if they are completely inactive.

Once we see the overall vested SP tend towards 90% ish - we'll see different behaviours with regard to powering up and down.