Audiophiles often upgrade equipment as new models or technologies emerge. The Boulder 866, while advanced, sits in a segment where buyers frequently rotate gear to pursue incremental improvements or different system synergies. Sometimes people call this a hobby while others call it a disease.
As you noted, the high number of used Boulder 866s likely reflects normal turnover among audiophiles, the amp’s high value retention, and the active nature of the high-end audio market-not a flaw in the product itself.
I suggest you not worry about it.