The best safety money can buy and the one anyone should posses is the one between you ears which is the one safety switch on the exterior of the gun
This is a fine sentiment, if only it always true.
The problem with the Mk-I Organic Safety System is that it is prone to failures from multiple sources. Adrenaline, fatigue, distraction, anger, impatience, etc.
Most of us have seen the video of the dreadlock-wearing DEA agent shooting himself in the foot with a Glock. And that was a low-stress incident. Around these parts, a local officer was going off duty, changing in the locker room while telling a rookie why he chose the Glock to wear in his S.O.B.* holster. As a lefty, there was no safety to worry about "bumping off" in contact with his body and *Boom*! He sent a .40 Caliber JHP out the bottom of the holster through his right buttock (that's
beyew-tock for you Ensign Pulver fans) into the locker room floor. Why did that happen? The end of a 10-hour shift for an officer who began the day badly fatigued because he and his wife had a new baby at home. Plus his day included several adrenaline rush moments that added to the fatigue. Just a momentary lapse of situational awareness and a misplaced finger nearly ended an 8 year career.
If the police departments are getting rid of them, does that mean that there is a problem or are they just improving to a better weapon?
Usually used police guns are "traded in" to the supplier of new guns for credit to help purchase newer, more ergonomically designed or improved guns. For example, an agency might trade in S&W 4006's to Sig for new pistols, reducing the cost of the Sigs. Sig, in turn, sells off the guns to distributors to offset some of the "discount" they gave to the police agency. Marginal guns, when inspected are turned into spare parts. S&W refurbishes trade-ins and sells them to distributors.
Most police guns are "lightly" fired. Figure 200-250 rounds/year for many agencies up to 1000 round/year for qualification.
*S.O.B. = small of the back style holster