Clearly, "theory" says that an aluminum tube subjected to the sorts of bending and flexing stesses that bike frames and forks are under should fail rather rapidly. But, in "real life", a failure of an aluminum frame or fork from routine riding stresses is so rare that many experienced bike techs don't remember ever seeing it occur.

The folks who design and build aluminum frames and forks OVER-BUILD them to a very large degree to prevent such failures. Of course, the result of that is that many aluminum frames weigh almost as much as the best steel frames...beefy up the aluminum frame cancels out the supposed weight advantage of aluminum over steel.

Where steel has its BIG advantage over aluminum is under traumatic stress. Run into a curb at high speed with a Reynolds 520 or generic 4130 steel fork and frame, and a skilled tech can realign them as good as new. An aluminum fork or frame that is bent or twisted out of alignment can not be safely realigned (and Reynolds says that heat treated steel such as Reynolds 853 should not be realigned either).

If someone wanted a frame and fork that will last a lifetime, there was never anything better than Reynolds 531 or generic 4130 frame a good supplier...but who really wants to ride the same bike for the rest of their life?