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Because there will be less SP being generated as the interest rate falls. This reduces dilution which is generally viewed as a good thing.

High interest rate, high dilution
Low interest rate, low dilution

These forces are a yin and yang on each other. Both cases are an incentive to hold if you believe in the social media platform. If you don't believe in the platform, it would be wise to cash out. STEEM is nothing without it.