The advances in safety technology are probably the biggest argument against this in my opinion, however I do believe materials degrade.
Some years ago, I picked up a Peg Perego seat from Craigslist for a friend who was using a long-since expired 3-point infant seat which was given to her after her car & car seats were destroyed in a house fire. Yeah, I know, a Craigslist seat is problematic anyway, but I wasn't as aware then as I am now and it was an improvement over the seat which not only had an unknown history but ALSO was expired. Anyway, the person I got the seat from was a Grandma who told me (and frankly I believed her, I think the seat was only $15 or something so she wasn't making a to n of $$ off it) that she'd used it for her grandchild 3 or 4 times and it had been stored in the hall closet the rest of that time.
My friend used the seat for 6 months, at which point her daughter outgrew it and she stored it in a closet also. We're talking a VERY LIGHTLY used car seat in outstanding condition.
When I got pregnant with my daughter, she gave it back to me. At the time when she gave it back, it was not yet expired and I thought I'd be able to use it before it expired, assuming a 6 year expiration. Well, then I found out that Peg seats expire after 5 years, and this one was due to expire about 2 months before my daughter was born. So I went out and bought a new seat and put this one in the corner in my bedroom figuring I might use it occasionally for non-vehicle purposes (a place to lay the baby for a minute and that sort of thing). At that time, it was in perfectly working condition, all the buttons and pulls were smooth, etc.. A few months after the expiration had passed, I decided I was going to take it off its base and see if I could talk my daughter into sitting in it and rocking a little now and then so I could maybe eat without holding her all the time (she was a high-needs newborn). Anyway, the seat would NOT come off the base. It took 3 adults to pull it apart. When I was finally able to examine the mechanism, the problem was that the plastic in the release mechanism had become too pliable and was no longer exerting enough pressure on the latches which hold the seat to its base.
If the plastic in the release mechanism was no longer working as designed, what about the plastics in the other parts of the seat? Needless to say, this seat was NEVER used in a vehicle after its expiration. I did have some luck using it as an infant chair in the house, though.
Keep in mind that this infant seat was less than 6 years old and in otherwise PERFECT (looking) condition at the time when I discovered the plastic had lost its integrity.
I have also developed quite a respect for expiration dates along with my hobby of recycling seats. I've found broken plastic supports on interior parts of car seats or places where there are cracks around rivets and bolts due to aging plastic. But this has been on much older seats, not ones just a few months past expiration. My Peg example is the only one I have of a seat which failed so soon after its stated lifespan.