Come on, you don't think manufactures using all these plastic pieces on the engines now don't precisely know that they are going to fail in x number of heat cycles? I guarantee they know exactly how long the car will last with average use and at what point it's more expensive to repair than it's worth. I think you underestimate the automaker's bean counters and exactly how much obsolescence is built into the auto industry.
They don't use plastic because they know it's going to fail (besides, just because it's plastic does not mean it's guaranteed to fail). They use plastic because it's cheap as fuck and they save lots of money on machining time.
I work in the manufacturing industry, and my company builds products to last a lifetime. All of our products are overbuilt with all metal and minimal plastic. That said, nothing lasts a lifetime, not even metal parts. Everything needs maintenance, and a small portion of all complex machines will fail for some unforeseen reason. You try to cut costs where it seems appropriate to minimize cost of goods and labor while maintaining efficiency and planned lifecycle, and sometimes it works and sometimes it doesn't. That's just the business of manufacturing, and auto manufacturers are no different.
That said, if you're unconvinced and unhappy with the way Volkswagen operates, you always have the option to switch to another brand. There's nothing stopping you from voting with your wallet.