My aluminum '89 Cannondale road bike is getting close to the 150,000 mile point so I hope it will last well beyond that (but it still has a long way to go before hitting 40 years). Yes, all bike frames will eventually fail if they're ridden long and far enough but my impression is that most are either crashed or thrown away long before they get a chance to fail from fatigue. I agree that the warranty is based on market data showing that very few bikes will still be in use by their original owners by the time that fatigue becomes an issue.