Well.......I can tell you from experience when my dept dropped the 5906/6906 in 2013 the driving factors were:
1. Cost of new guns should a whole gun need replacing.......$1000 a gun! At that time S&W no longer had a dedicated Gen 3 production facility. "New" Gen 3's were produced once a year at a factory that normally produced handcuffs, but was retooled for an annual pistol run.
2. S&W statement that new replacement parts would no longer be manufactured and what was in the pipeline was it.
3. We were seeing guns, the 6906 at a higher rate than the 5906, have parts failures that dead lined the gun beyond recoil/trigger spring replacement. Mostly failures of the sear release lever (disables the decocking function) and extractors worn to the point that the effected guns were experiencing failures to extract about 5%-10% time. The guns were an average of 7 years old.
The idea that most police pistols have little use is more internet myth that a reality. In every department there are a few guns, mostly at the administrative level that are essentially NIB after 5-7 years (these are the ones that get the internet bragging, lol). Then there are few that are very well worn (the G17 on my side right now is 7 years old has 29,545 rounds on it as of Dec 31). Most are somewhere in the middle, with 3000 to 5000 rounds on them. Within the next 3 months we will replace every pistol in the department. 5 to 7 years in the normal interval for all but the very small or very poor departments. It's by design, because at the 5000 round mark it's time to overhaul every pistol, new springs (recoil/magazine/extractor/trigger), firing pins/striker, extractors. The cost of doing that on a Glock is about 75% of the cost of a new pistols when the old guns are traded one for one. So away they go