2 years is based on the shelf life of the polymers and some dodgy tale about skin acids . We use similar polymers in pump hoses. If it's on the shelf for three years it gets binned because some customers send them back. If it's been on the shelf 35 months, they get used and have exactly the same service life as a brand new one. Most hoses fail because they are simply worn out, but a few are misused and fail sooner. When we are short of stock we can offer expired ones, we don't see them in the warranty list any more often.
Same goes for a helmet IMHO, in use the clock is ticking, in a dark, cool cupboard it slows down. 80% marketing 20% fact IMHO, they don't explode Logans Run style the second they hit 2 years old.
Andy