Just my non-technical expertise.
Granted I don't know much about engineering but I have been the end user of "plastic" (fiberglass and carbon) in both sailing and bike racing.
These are both very sophisticated products designed to have very high strength to weight ratios. In both cases Aluminum was only a fad. All high end boats very quickly went to fiberglass and now kevlar/carbon. And as far as bikes you would never catch me on an aluminum bike. I went from Steel to Ti to Carbon and skipped aluminum due to fatigue issues while racing. Granted a good bike will cost you $4,000+ the difference between a good aluminum and carbon bike is not that much different.
1986 Sovereign 25' CB
2007 Chevy Silverado 1500
2007 Toyota Land Cruiser