I'd consider carbon, but I want this frame to last 20+ years
Why would this desire rule out carbon?
It may be less crash resistant than metal but in terms of fatigue resistance -- crabon beats any metal frame. I personally switched to carbon because I got sick of wearing out metal frames.
I should also note that carbon is the most repairable frame material and crash resistant crabon frames are available:
http://calfeedesign.com/products/luna/