My agency transitioned from the 4006 to M&Ps several years ago. The agency needed to procure more handguns, and the 4006 was basically out of production.
The 4006 is a fine handgun, but the M&P (basically a Glock clone) was well-received by officers. It was cheaper, lighter, ergonomically superior and frankly, I shoot a little bit better with it.
We had the opportunity to purchase our 4006s, and most officers did.
There was an issue with the M&Ps having a malfunction associated with the mounted taclight, and we were reissued M&Ps with several small modifications (notably a stiffened frame). However, the M&Ps have been overall very reliable and require even less maintenance than the nearly trouble-free 4006.
So, I think the 3d gen Smiths were great guns, but they just can't compete in terms of price with the polymer frame guns, which, in the case of S&W, continue to benefit from incremental improvements.
In the hands of an individual shooter, the difference would be negligible, and the 3d gen Smiths deserve respect as the end of a nearly 50 year evolution.