I think carseat expiration is due to several reasons, not just plastic degrading. Here are my reasons on why I think carseat expiration dates are valid.
1. Plastic does degrade over time. It doesn't magically happen at 6 years, but it is a good date guideline, especially if a carseat has been used in a hot car or stored in a hot garage.
2. Manufacturers make safety upgrades to seats all the time. They want to get older seats that might not meet the new safety guidelines out of the market. Seats made 10 years ago, often had a 20-22 pound rear facing limit on them (this was for convertible seats, not just infant seats). Now, those convertible seats rear face to 30-35 pounds, and are much safer. Older seats used 3 point harnesses or shields to protect a child, and now more seats are using safer 5 point harnesses on seats.
3. After a certain period of time, seats loose manuals, stickers wear off, harnesses get frayed. It is hard to get replacement parts for older carseats. Without good working parts or a good manual, carseats may not be used correctly.
4. Manufacturers are except from lawsuits from an injury that might happen to a child in an older carseat. If a seat fails after it is expired, you were the one who took the risks of the plastic degrading or possibly having a seat that fails newer safety regulations. You cannot blame the manufacturer for using an expired seat when they told you not to use it.
Maybe Britax is working on improving all their seats to 9 years, but since some of the seats have 6 years still, they aren't in a rush to change them. Britax may also feel more confident about their seat design now and don't anticipate any safety changes soon which would cause them to want to get older seats off the market. I would love to see seats have 8-10 year expiration on them. I think that would aid some families who need seats passed down to other siblings. I want to make sure the seats are safe to use up to those dates first.