You don't sound ridiculous. I've heard that sun exposure breaks down the foam over time and that they recommend a new helmet every 3-5 years. That means if the helmets were in a storage room, you're better off than one that had been on display by the front door, and that helmets that are riden more miles on will have a shorter lifespan, I guess. I intend to buy a new helmet every 5 years either way, but I'm sure there's a 20 year safety buffer in there somewhere (I see people on 1980's helmets that haven't visibly decomposed yet).

I am morally against using a shop's service and showroom space in order to give the sale to a different company, but you do what makes you feel right.