My guess is that the push to make car seats expire has more to do with profits than anything else. Here is an example: Although most infant car seats now come with a 5 pt harness, the 3 pt harness is just as safe for an infant seat;however, it was something that set the seat apart for a short while a few years ago and then all the manufacturers jumped on the idea. Car seats are not something parents buy a lot of or will need to replace often.
Infant car seats, in terms of technology, haven't changed all that much over 20 years. The base was introduced in the 90's and actually using the base is not considered as safe as by some as not using the base and strapping the seat in directly to the car.
As far as the plastics degrading, that information used to be on carseat.org but I am guessing that the Plastics manufacturers got ticked off about it and made them remove it. It wouldn't really make sense to create a life saving device from such cheap plastic that it could degrade from the basic elements - sunlight or cold weather.