I think beacon mint incorporates the idea behind this suggestion, but instead of relying on the balance in a wallet, which as brod mentions can be manipulated, Beacon Mint considers the amount owed with relation to time. This means that as a user with high mag and low balance reaches a higher reward owed to balance ratio they will be more likely to stake, but this ratio will have no effect on the users likelihood to stake. It's easier to manipulate reward owed over balance than it is reward owed over time.

An issue with Beacon Mint I think we will need to explore is whether we can make it so people with low mag and low balance -- most new users -- can still stake in a reasonable amount of time.