I saw a study years ago, conducted by the military on frame material longevity. Steel frames had the highest longevity, polymer next and in last place was aluminum alloy. I don't remember the guns tested except that the steel frame was a 1911. Neither do I remember the exact round count, but steel frames were in the neighborhood of 500,000, polymer frames were around 300,000, and aluminum frames were less than 100,000. Now, I know that 100,000 rounds is a lot and almost no one will wear out an aluminum alloy frame, but there is another way of looking at those figures... An alloy frame is 5 times more likely to break than a steel frame, and three times more likely to break than a polymer frame. Aluminum is strong, but also hard, therefore brittle. The most likely failure mode for aluminum is a cracked frame. So, since I can no longer find the source study, you can consider this apocryphal, but this is the basis of my reasoning.