The tensioner pads wear simply because they're made of plastic and are pressing against a steel chain running over them at considerable speed. It's designed obsolescence - almost all of them made it through their ten year warranty and they're now exceeding their 15-year expected lifespan. Audi don't care if they all explode at this point.
The pads are made of nylon 4.6. If they wanted them to last longer, they could have used a different material like UHMW polyethylene, Acetal, even PTFE, and provided an oil feed to the wear surface to further reduce friction. This would have been more expensive though. Nylon 4.6 lasts longer than the warranty so it will do.
Once the pads are sufficiently worn, they crack and fall apart whereupon the engine shreds itself. A loose chain slapping on worn pads makes them more likely to shatter, but is not the cause of the wear in the first place.
I replace the pads on non-rattly tensioners to avoid them shattering, but the tensioners will still wear out eventually.
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