What response would you like? Without significant cost and research with crash testing, it can't be proven either way. Like many conspiracy theories, it would likely never be proven or disproven to the satisfaction of those making claims on either side. Could expiration dates be arbitrary? Could they just be for liability reasons? Could car seats last longer? Sure. Are there risks involved in using an expired secondhand carseat with an unknown history? Also, Yes.
The data you want could also come from the author, who, instead of making unsubstantiated insinuations, could have taken a statistically significant number of USA-certified car seats that are well beyond the expiration date and then subjected them to typical crash forces on a crash test sled and then a second time to see if there is any difference in performance compared to a new model or if any samples exhibited breakage. Yes, this costs money, but we're not talking about plastic patio furniture here, we're talking about due diligence in regard to a safety device designed to protect a child. We're also not talking about the longevity of one specific part, such as the plastic shell material or harness straps or metal frame only. We're talking about the integrity of the entire product that is necessary to protect the child during peak crash forces that can can occur in an impulse of tens of milliseconds along with the labels, parts and instructions necessary for it to be used correctly.
If an author or researcher is genuinely concerned about safety of children when making such allegations in a book sold for profit, isn't the burden of proof upon them? Scientific method could and should have been used by the author to test their own hypothesis, rather than implying it is some industry wide cover-up and indirectly encouraging others to take possible risks with their babies because the author couldn't find any existing research by sending out a questionnaire. Car crashes are among the top few killers of kids at that age, so it seems a sensible to take this next step, don't you think?
One could also ask if the author (or really anyone who believes this is some grand conspiracy) actually uses crashed, expired car seats for their own infants. We know from past experience that researchers profiting on books with related claims did not put their money where their mouth is, so to speak, at least not with their own children. Honestly, if I was making a claim like this for a book, I'd have started with visits to UMTRI, CHOP, NHTSA and IIHS here in the USA and talked to the independent researchers there in person, rather than rely upon mailed questionnaires to child safety seat manufacturers. Even if they didn't have data and also did not have any references for more information, they could have explained scientific method and how it could be used to answer the question.
Hey, if you or the author are willing to fund the necessary research for publication in a peer-reviewed scientific journal, I'm also very interested in the results. Heck, bring me on as a consultant! It would be great to save parents money, save the environment AND save lives, IF the author is right.