Deontologist

Categorical statements tend to be wrong. Carbon frames don't conk out after 3 years unless something's happened - like an accident.

As a material, carbon fiber composites last basically infinitely long as long as the applied load is about 60-80% of what its ultimate strength. So if you have a tube that can withstand a maximum of 100 Newtons in compression, you could cyclically load it with 80 N and have it last basically forever (as in millions and millions of cycles).

Bikes - collections of tubes - are more complicated but I think it's safe to say that carbon is a durable material if it hasn't been damaged in a crash. Even so there are repair options for carbon, while none exactly exist for other materials such as aluminum. Dent, crack, or otherwise significantly damage an aluminum tube and there's no going back - there's almost no repair option.

Carbon, by its nature, can easily be repaired. It's just a collection of fibers bonded together. Repairing it just means removing the damaged fibers and replacing them with good ones. That's a relatively easy job versus repairing aluminum. You can't just remove the damaged aluminum and replace it with good aluminum. Aluminum alloys are crystalline in structure and it's harder, as one might imagine, to manipulate structures on the crystalline level by hand. Impurities in the crystal structure can greatly modify the properties of aluminum ... hence aluminum alloys!