I like the proposal, the earn rate will inevitably go down at some point, and this looks like a reasonable attempt at making it somewhat graceful.
A few questions
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Is the 1.5% ceiling on the periodic rate change expressed in terms of percent or percentage points? I.e. 1.5% of 20% is 0.3 percentage points. Depending on what you meant the resulting rate is either 18.5% or 19.7%.
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What happens when the yield reserve goes to 0 faster than the rate can adjust and prevent the drain?