I think there is an 800lb gorilla in the room, but you're painting it as a raccoon.
What you are outlining is what we call engineering changes. Steyr and other military grade manufacturers don't just go switching materials just for the sake of amusement. Likewise, they cannot go substituting materials without proper T&E, especially when there's lives on the line. Materials selection is likewise a huge ordeal. Metal ain't metal, and plastic ain't plastic. There are numerous material properties that go into the selection process and those characteristics effect the outcome.
Over the period of 40 years, Steyr has made engineering changes; no one disputes that. They have gone through the process of materials selection, testing, evaluation and ultimately roll-out. If you ask me who I trust in terms of engineering T&E and R&D, it's Steyr, not an aftermarket parts supplier.
I work for a company that manufactures industrial machinery and I can tell you in no uncertain terms that we don't just go swapping materials and dimensional specs without an exhaustive process. More often than not, things will seem okay at first, and then go haywire. If any of the trigger "upgrade" companies have data to verify longevity, then that would go a long way to convince me. As it sits, I'm not at all convinced, and apparently I'm not the only one.