Ok, let me clarify. While a credit sink does not need to be universal, I believe that they work best when they are universal and not voluntary. This is my personal opinion.
Put in a life tax. Every day the game destroys 0.05% of everyone's credits regardless what they do. Poof, credit sink issue fixed.
EDIT: Thing of it is, we don't even know if "credit sink" is the reason the DEVS did this. THEY ARE NOT TALKING ABOUT IT!
It sure would be nice to have the rationale, aside from, "Durr... we found a bug on our list and fixed it with absolutely no critical thought and even less testing."