Sorry about the crack on your degree.

I still don't buy it. So there are low-end kids bikes built with big stiff tubes - so what? There are also Huffys with drop bars. The use of Tubing A in a kid's bike doesn't really imply that it was necessarily built that way to guard against breaking... though of course, in this case, we're probably talking about pretty crummy aluminum alloy. Tube sizes used for aluminum bikes are mostly a matter of style. Increase the wall thickness and you can flex all day long with no problems on an appropriately designed aluminum bike. You still haven't addressed the use of aluminum in structures that are subject to high-force oscillations that are used for decades, nor the fact that the failure of Vitus frames at bonded joints does not suggest that the material used was problematic.

ETA: It is reasonable to suggest that most people don't want to bother with aluminum if the result is a frame that is whippy and not all that light. And that would be a good point. The lightest possible aluminum frame is going to be pretty stiff, for the reasons you suggested. It would be surprising if aluminum bikes made with more flexible, heavier tubing were more widespread. The point I'm making is that the use of mostly stiff, large-diameter Al tubesets does not imply that this kind of tubing is necessary for a fatigue-resistant bike. They're not. There's just not any demand to speak of for flexy aluminum bikes.

I've forgotten to add, by the way, that the question of aluminum ride quality has gotten a bit lost in this discussion. I think that the "harsh ride" of aluminum is way overstated. My aluminum race bike doesn't ride any harsher than the average racing bike I've ridden, built of any material, big tubes and all.