[dme123]

The reason they don't offer longer warranties on aftermarket parts is because cars over 3 years old are more likely to suffer damage to one part through an unrelated part (your tensioner issue), or owners put additives in the coolant that might damage the pump. Or parts are often ineptly fitted, leading to premature failure.

You've basically answered your own question as to why they don't offer longer warranties on parts for older cars! When you DO have a warranty on a part you often have fun and games proving that the part failed due to a fault rather than being damaged by something else anyway.