I feel this "December of the 6th year" thing makes it very difficult to explain the importance of expiry to other parents. People say all the time "oh that's the stupidest thing I ever heard. It's not a gallon of milk."
I get what you're saying, but like you've found with the "stupidest thing" comments, I find that telling them the seat is going to fly apart and not hold your kid in doesn't convince most people. Of course I do believe at some point after the expiration date, that becomes true (and nobody knows when that point is for a given car seat), but I have had better luck pointing to the recall list and saying something like:
"Look at all these car seats that have been recalled. Some of them get recalled because after a year of use or after three years of use the harness adjuster stops holding tightly enough to keep the kid restrained in an accident, or the buckle starts falling apart, or the chest clip starts disintegrating and posing a choking hazard, or the whole seat design is found to be unsafe in certain circumstances, or what have you. When those kinds of problems start cropping up after a year or two or five, the company issues a recall, notifies registered owners, and puts out a fix or replaces the seats. When those kinds of problems happen after the expiration date, the company has already washed its hands of that car seat and will no longer track, notify, or fix problems as they occur. So using an expired car seat is as bad as using a recalled car seat because you have no assurance that it's still holding up."
I can't count the number of times I've had people roll their eyes at the mention of a car seat expiration date only to say "Oh, I never thought of it that way" when I explain it like this. Acting like a seat magically stops protecting your child at midnight on a certain date is bound to register on a lot of people's BS-O-Meter, but explaining that companies only agree to monitor the continued performance of their car seats for a limited amount of time makes a lot more sense.
Of course plastic and straps and things also degrade, and that's a factor too but it's only ONE factor.