So here's the pivotal question: what is the service life of an aluminum bike frame?

I'll grant that a steel frame will last longer. Properly cared for and not damaged in a crash, I wouldn't be surprised to see a well made steel bike last 100 years. Of course, that isn't much of a benefit to me personally since I won't last that long.

Will my aluminum bike last 20 years? I'm hopeful I'll still be cycling then, although I may have to go to the retro-grouch LBS to find a 1-1/8" headset and an English threaded bottom bracket.

Honestly, I'm not sure this question is answerable, because I think the life of a well-built aluminum frame is longer than the time since the bicycle industry started producing well-built aluminum frames.