Car seats in Europe don't have an expiry date. The US expiry policy of six years is part brilliant marketing by manufacturers and part paranoia.
In Europe we recommend to not use car seats longer than 10 years. This has nothing to do with plastic or anything else on the seat degrading. It's related to other factors such as lost manuals, lack of spare parts, changing standards, knowing history of the seat, etc.
Another excellent travel seat for forward facing kids 4+ is Britax Adventure. Also called Highliner. It's a high back booster seat which is light weight, comfortable, cheap and has smart threading of the seat belt. Weight of this seat is less than half of Evolva.