I understand what you're saying and I too prefer steel but the metal alloy in the GSG MP-40 has been in use for well over 40 years in other rifles. In 1988 I bought an Italian made .22 copy of the Israeli Galil rifle and it saw extensive carrying and pest control use in the bush on the Bruce Peninsula. Most of the receiver and the front sight etc. were made from this same alloy and they wore well under hard use. I did have to touch the black paint up in places but the metal itself did not chip or break. The areas on the rifle that required extra strength (bolt, barrel, butt plate, magazines, all screws and bolts etc.) were of course made from good quality steel.
One thing I remember about that rifle was that it was 100% reliable no matter what ammunition I used. It was also very accurate and the action stripped down in seconds for easy cleaning.