In fact, breakage at bonded joints is a common problem for older frames joined in this way, irrespective of material. Old bonded CF bikes have the same problem, but because no one thinks that CF fatigues (though of course it does), they blame the real culprit: the joints. You are grasping at straws.

You appear to know about as much about materials science as the ancient Egyptians. "...no fatigue limit, so the material is prone to cracking." Okay there, smarty-pants. One of these things is not equivalent to the other. Aluminum is used all the time in applications under which it undergoes considerable flexing forces. Ever look at the wings on your airliner when passing through turbulence? Now multiply that by near-daily flights over service lifetimes of often 15-20 years and tell me again that designing aluminum structures to be stiff is the only way to be certain that they will not fail from fatigue.

And please - waving an engineering degree around to shield your ignorance from criticism is crass. Being an engineer doesn't mean that your gut feelings are more likely to be correct than the average joe's.

As for the tubing sets sold by manufacturers and used by builders, there's a much more parsimonious explanation for their tendency toward being large in diameter, thin-walled and stiff. High-end aluminum is in the greatest demand for racing bikes. The frames that most people want for most of these applications are light and stiff. And so the tubesets are built accordingly. But there are other uses for aluminum, and lots of non racebikes have been built with it. And they are built with tubing that is thicker-walled and smaller in diameter. Lo and behold, it flexes AND is plenty strong.

That, by the way, is your answer, though you've been too busy waving your hands and diploma around to see it staring you in the face. The lightest possible aluminum frame does indeed require stiffer tubes in order to have sufficient strength. Though it should be said that even this basic rule isn't as clear-cut as you might think. The 1,000g Spooky Skeletor has smaller tubes and a much better ride than the old CAAD3s, but it is still much lighter. But there are a lot of bikes out there made out of aluminum that are not race bikes, and they need to ride softer. At the same time, weight is not as much of a concern. So these bikes are made with smaller tubes with thicker walls. But the reputation of Al as a material was built on Cannondales CAAD racing bikes.

So what I said was true: most people think that Al is harsh because most people experience it as a road racing frame. Road racing frames are supposed to be stiff and as light as possible, and Al is well-suited to this, so you get a lot of stiff, light frames. That doesn't mean, though, that there is any need to make an Al frame stiff in order to make it safe. It is not at all necessary. Any claims to the contrary are ignorant storytelling.