Hi Rudi. On the surface, that sounds reasonable. But from another viewpoint, you might consider the silver to be an impurity, yet its concentration doesn't keep building up (it would if we didn't shift and dump tanks). If the impurities come from the paper, they ought to sort of track with the silver concentration.
Another way to consider this is to ask, "What could possibly cause a buillt-in impurity to become more concentrated?" Provided it doesn't settle out or stratify, evaporation is the only possibility I can see. So, aside from a slight amount of evaporation, it doesn't seem like any built-in impurity CAN increase in concentration.
A not-so-obvious way in which the impurities are removed is via solution carry-over to the wash.
Anyway, I'm not convinced that this is a plausible reason. Maybe I'll try calculating it out later tonight.