I think you missed the part where I've seen them doing their testing for their research with my own eyes.
Now, whether or not you believe me? You're asking for our help, with the understanding that what we say may sway you.
They do quality control testing for X number of uses. It breaks after some thousands of repetitions. They know what that average number is, based on environmental issues, child use (soda poured over it once a month! Sure, let's test that), parental fat fingers, etc. Based on that, they've declared their expiration date.
So yes, they completely have, highly controlled experimental and testing situations.
It sounds like you don't want to believe it. Ok, up to you. Replace your seats more often than necessary, since otherwise you're using your child to do your own testing, which is not recommended. I guess I'm just not seeing the downside of replacing a seat when you can probably go through the carseat life of your child (about 13 years) paying $300 total if you shop well. $100 at birth, $100 to go from about one to six years old, and $100 for a booster for the last several years. You never run into expiration dates, assuming one child is using the seat there, since your child will make a change to the next stage anyway before it would happen. You don't have a seat that's five years old and looking like a good fire would be the best way to clean it anymore after that, too.
Wendy