How many kids die in MI due to being left in a hot car each year? It's not like the sun doesn' shine there during the summer, and it doesn't get blazing hot inside your vehicle. Besides, can't extreme cold also damage plastics like extreme heat does?
My :twocents:, climates that have both extreme heat and extreme cold, are going to cause more wear to seats from the constant changes in temperature than a strictly hot climate would.
We have consistent -20 celcius temps in the winter, but heat up the vehicle to a good 15 or 20 above celcius if it's running long enough while we're driving around. That is a big variance in temperatures - then there's the really hot to cooler temperature that happens in the summer when the air conditioning is turned on. We get up to 30* celcius in the summer, sometimes a bit higher than that... so you can see the vast extremes that car seats in this area go through regularly. I'm guessing MI would be similar, although not having quite as cold of temperatures for as long as period of time in the winter, so it's safe to say the seats there would be more prone to wear and tear due to temperature changes than a seat being used in Florida would be.
Most manufacturers do extreme temperature testing on their seats to ensure that it stands up to the repeated extreme exposures, but this is a case in point of why manufacturers give expiry dates that may seem conservative to some people. They need to give a time frame that they know every seat will still be safe. Because of the variety of conditions - weather, humidity, exposure to UV rays, use of the seat etc, it's impossible to predict which seats would continue being safe past those first 6yrs.
And I think that's the most important message when it comes to expiring seats - there's no way to know the moment a seat quits being able to protect the precious occupants using it. By following the manufacturers expiry date, you're only using the seat within the time period the manufacturer knows the seat will continue to protect it's occupant. Go past the expiry date & it's like playing Russian roulette - and no parent wants to be the one that has the seat that fails to protect their child.