To answer the OP's main question, I don't think that the "quality" of Glock has gone down hill. We have Gen 4 G19 and G17 at my job. They work fine. Much ado about nothing is made about MIM. I have several guns with MIM parts. They work fine. Gun makers run in to problems with updates in designs that have narrow performance windows. Meaning parts, subsystems etc have specified sizes. There is a specific size that every part needs to be. Since guns are complex machines, they cycles of function in which everything has to happen within a specific time. There will always be a difference in what it is supposed to be and what it actually is. Some guns run with big differences in things. Some are not tolerant of much variance at all. I will use the mighty G Lock as an example only because that is the gun that I have the most professional experience with. Glock's change in manufacturing method and design spec for extractors resulted in a slightly different sized and shaped extractor. Think late Gen 3 and Early Gen 4 9mm Glocks. Add a little wear or a faster extraction due to spring wear and a once functioning gun became an intermittently functioning one. Another would be the G22s reliability, or lack thereof, with a WML. Same as the reliable G17 in design and parts. But, stiffer frame due to WML installation = higher slide velocities = guns out running their magazines = weapon malfunctions. This was the theory at least and some G22's ran well with lights. The G22 has a narrower performance and is less tolerant to changes in timing. Less to do with quality than manufacturing and engineering.
With regard to the trigger(s) on your M&P pistols, have you checked out the Apex duty trigger kits? Los cost and they really make the trigger feel et through "glockier".
Doctor Gary Roberts has posted some excellent standards for reliability for pistols in other forums. They are, in my opinion, a solid benchmark that a defensive pistol in any caliber should be able to meet.
