Nothing is ever that black and white in baseball. Bruce was terrible in 2014 and 2015. Saunders has no meniscus in his knee and didn't play last year. It is certainly not impossible that internal analysis would lead a FO to believe that they'll get more from Jay Bruce than from Saunders. Once they've reached that conclusion, why not make the move?
I don't think anyone is suggesting that it would be acceptable to move a high end prospect from the system, but if the cost is like Travis Bergen and a couple million dollars to roll the dice on someone they have reason to like, it's silly criticize it.