There's no evidence that used, but uncrashed helmets are any more dangerous than new. In this study:
"Foam cores were extracted from 63 used and unused bicycle helmets"
"Based on these data, the impact attenuation properties of EPS foam in field-used bicycle helmets do not degrade with the age."
"Based on these data, old helmets should not be discarded based on an incorrect assumption that their impact performance has deteriorated with time."
The only variable that was "impactful" was the density of the foam.
Furthermore, per hour of commuting, you are actually more likely to get a TBI riding in a car vs biking, 0.41 vs 0.46 per million hours traveled. I have yet to see anyone wear a helmet in a car, which makes me think that most people are worried about bike helmets for cultural reasons rather than doing an honest risk assessment.
This study goes on to make the stunning conclusion that:
The benefits of cycling, even without a helmet, have been estimated to outweigh the hazards by a factor of 20 to 1 ... Consequently, a helmet law, whose most notable effect was to reduce cycling, may have generated a net loss of health benefits to the nation. Despite the risk of dying from head injury per hour being similar for unhelmeted cyclists and motor vehicle occupants, cyclists alone have been required to wear head protection.