What happens when you don't have any coins left? For example, what if someone deposits 100 BTC (which is extremely unlikely at the moment, but say if) and expects to earn 1 BTC every month? Can you handle it financially? As far as I'm concerned, you have not disclosed the amount of personal stash you have. If you don't want to do that, which is very reasonable for a service like this, then perhaps limiting the amount is a wise choice.

So basically what you're trying to do is take loans from every depositor.