I have a hunch that the sustainability of a bike has a lot to do with the commitment of the owner. Keep it maintained, don't let it deteriorate, prepare to find a good home for it if you get a new bike. And ride it. Find ways to eliminate car trips.
The more exotic a bike, the fewer made, to the point where the consumption of materials such as carbon fiber probably doesn't matter very much. Cheaper bikes that are made in larger quantities still tend to be made from mostly recyclable materials.