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It isn't necessarily about keeping SBD at $1. The whitepaper states that the total value of SBD should not exceed 10% of the total market cap of Steem. You could have SBD be worth $10 USD if Steem was $100. And even if it was $1, why would people power down? The SP would allow them to accumulate more SBD over time.

In a period of market fluctuation like we've seen in the past the price did not remain high for an extended period of time, both the last time and this time we've seen a lot of powering down going on and cashing out of steem, but this time SBD was higher so voting power mattered more during this pump phase so more people stayed locked up in SP.