That's the point of a warranty though. To cover you if it does go bad. Your right, in a perfect world you don't need a 10 year 100,000 mile warranty on your car. But that doesn't get extended every time to take it in. Anything manufactured has the possibility of failure, from a pencil to a submarine. The warranty is coverage for you in case it fails.
However, business can't happen if we only sell them one thing per person in an area. After a time we no longer are making money, we are providing free parts and sometimes labor to people. A battery is a consumable, it isn't designed to last 10-20 years. Most modern cars your battery will last 5-7 years. Maybe more if you never abuse it. Blame the car manufacturers for this. They are designing cars to use the battery more often for more things than they did in the past. Batteries I pull out of cars from the 60's through the early 2000's are usually much older and last longer than the ones I pull out of modern cars. Because of this we can only guarantee they will work for a few years. If we constantly renew a three year warranty on a car that will shit a battery every 5-7 years, we run the risk of never selling another battery.
It goes the same for hard parts. Yeah, you exhaust should last quite a while, but sometimes it doesn't. And in the case it doesn't we give you another part for free. It doesn't make good business sense to continue to give you free parts for the rest of the cars life. Cars themselves are consumables. They aren't designed to last forever. Almost every part has a shelf life, from your brakes to the engine. Again, this isn't my fault. It's the nature of the business of mechanical things. If we could make money handing out free stuff I'm sure we would. Customers would flock to us. They'd hear about how we will only charge once then the rest of their lives they get replacements for free. You can't make money like that.
So what happens is that we offer a warranty that is considered fair. If the part fails before it's expected life span, we give you a free one. If you have any warranty left you continue to get free ones until that warranty has expired. Just like your car or TV or computer or camera. However, we can't keep upping the warranty every time.