This is ridiculous on so many levels. Maybe Apple should argue that the user's environment contributed to the problem. Where is all this dust coming from? Because I've had Apple computers for decades and never encountered a dust problem.
Also, what is the useful life of a product? Really... You see people coming back with 5 years old phones complaining about battery issues. Well, it's past it's useful life. Why should Apple have any responsibility for ensuring some arbitrary lifespan of a device. If it doesn't last as long as you'd like, don't buy it. A warranty is spelled out up front. One year coverage and you can pay for extra two years. After that, you're on your own. Don't buy the device if you can't accept this simple equation...