Originally Posted by GSXMike

You already answered your own question basically. Rubber breaks down from elements, chemicals, hot cold cycles, etc over time. The timing belt is not the only piece of rubber that is affected by those factors, all rubber breaks down over time unless you have it in a bubble with clean air circulating. All the rubber vacuum lines have tiny cracks that the naked eye can't see. Those are some of the first rubber items I replace. Pull one of your original rubber vac lines and fold it and look at it under light very close up and you may see tiny cracks. Air can escape through those cracks making your engine work harder than it has too. Just one example. Then you have the rubber motor mounts that are tasked to carrying the weight of the engine and dispersing the energy every time the engine shifts from throttle on/off. It's for the same reason why engine manufactures say to change your oil every 3 months OR 3k miles.

And I just noticed that you are in HI which is even more a reason to stat replacing all the rubber, because of all the salt in the air. Salt wrecks havoc on pretty much everything it comes in contact with auto related and more.