Other thing is you only get half of it back.
With new nodes receiving so much data, this can be a nice sum. And what we learned repair costs are considered low with the TTL that is set for the data.
If the sum that is held even after month 15 is high, this is an incentive to gracefully exit. Nodes leaving is not the best interest of Storj.
So maybe the periods must be shortened and percentage held reduced. Or the return after month 15 increased.