"Seems to me that most of the early changes were true engineering improvements. At some point (call it 1982), the changes were more related to cost savings or easier assembly.
The older revolvers did require more custom fitting by true craftsmen. More modern ones tend to be quite good mechanically but a bit more soulless and impersonal."
Somehow I never thought of guns having a soul, but "easier assembly" means cost savings. Also, use of modern manufacturing techinques rather than those from the 1850's, keeps costs down. Anyone who thinks that a Model 1899 (the first M&P) made today as it was then, would cost the same as a modern Model 10 is, to put it mildly, out of touch with reality. And has never seen the inside of a Model 1899.
Incidentally, when guns began to be made by forging, the technique was denounced as turning out mass produced junk because old timers felt that chiseling frames out of iron billets was the only way to make guns.