CPST here, I can answer a couple of the questions:
The car seat manufacturers say it's the last car seat you'll ever need to buy because they're marketing car seats and they want you to buy them.

They ARE getting away from that now, the boxes are coming out much more clearly marked...you no longer just see '5-100lbs' because people were thinking they could use the HARNESS to 100lbs and the harness was only rated to 40lbs.
Graco Nautilus; harness seat expires 6 years from date of manufacture (DOM), booster seat expires 9 years from DOM.
In general, most seats expire 5-8 years from DOM with the majority being 6 years. Newer seats, though, like the Britax Frontier, are being made with newer, stronger materials and are being given longer expiry dates, like 9 years for a harnessed seat. When in doubt, call the manufacturer's 800 # and have the seat handy to give them the model # etc info off of it. They will tell you definitely when the seat expires.
Before 2009 some seats were marked with expiry dates, some marked only with date of manufacture and expiry should be in the manual. For example, Britax seats expired 6 years from DOM.
All seats made in 2009 and later, though, will have the date of manufacture AND the expiration clearly marked on a sticker or stamped directly into the plastic by law.
I absolutely recommend car-seat.org for any parent or caregiver who wants more info on car seats. Many CPSTs like me as well as many knowledgeable parents are there to answer your questions.
To the op: You can tell your dh that the federal standards for car seats have changed at least twice (been made stronger) in the past 6 years.