The "stick" approach, where your buying power gets reduced overtime, is clearly a non-starter. I regret framing the idea that way, the "carrot" seems a much better way: Provide an incentive to spend the credits sooner rather than later. Say by a diminishing reward/bonus for doing so. Saving is fine, you can still accrue interest at market rates, but if you spend newly received inputs, you get a bonus for doing so, that bonus would rapidly reduce.