Is saving $100 worth ANY risk?
I'm a FF/EMT with 20+ years in the field and have seen some M/C fatalities and TBI's. Neither of these is a good thing!
I think it is stupid to argue about helmet replacement. JUST DO IT!
My thinking is this: if you replace your helmet every five years (even if based on "scanty" evidence), and it costs $500 for a new one (of course you can get a DOT/SNELL approved for much less $$), you are paying $100 a year to ride with a decent, clean, possibly state-of-the-art, allegedly safer, helmet. You likely spend more than $100 a year to buy other unnecessary items to wear or for the bike. Why would you NOT be willing to spend that on your head?
A clear conscience is usually the sign of a bad memory.