What about next week?  Why put each week on a different address?

Consider this, your Ponzi grows and becomes a billion dollar a week industry.  Do you know what would happen, sure your supper rich, but that's about the time some hacker breaks in and defaces you site.

The defacing won't be obvious, just an adjustment to the code that displays the address.  Perhaps they will simply steal the privet keys and disable the payment system.