My two favorite bikes are AL. One, is ALTEC2 and after 4 years and about 10,000 kms is as stiff as the day I bought it. Ditto my Ishiwata tubing double butted CrMo commute bike with 15,000 kms in the same period.

But I only weigh 66 kgs. A friend who a big powerful sprinter turned his Pinarello Paris into a wet noodle in 3 years. You could see the whole frame flex just by pulling on the handlebars.

These are just datapoints. Take the info as you will...