This already exists. If you dig and stake, your CLAMs increase by about 4% per month currently. In about a year they are increased by 50%. By not digging you lose 1/3 of that value in a year. That seems quite sufficient to me. This rate will decrease over time, but then the supply effect of even large digs will also decrease as the total supply increases.

EDIT: previously stated that half of value was lost per year by not digging but correct number is 1/3 (150 if digging 100 and staking for a year vs 100 if waiting a year to dig).