I have personally witnessed aged linear and high density cross linked polyethylene (LPE & HDXPE) chemical storage tanks break down over time and eventually fail.
Caveats: UV exposure is on a daily basis, all day long in many cases. Time frames were dramatically longer, with manufacturers often recommending replacement every ten years.
I have no idea how that material compares with the stuff our helmets are made of. I have no idea the amount of liability associated with severe head trauma resulting in death versus a chemical tank failing, floods a stream with nitrates and kills a bunch of fish but I assure you the difference will be significant.
I also have no idea how the above helps your discussion but think of it in terms of economics. Purchase a $1,000.00 helmet and use it for only five years. If you can actually afford this helmet you can afford to save $200.00 per year for the life of that helmet to pay for the next one.
PPE, no matter how expensive, has proven to be cheap insurance in my lifetime. You have to decide for you.