It is up to the consumer whether it would be better to spend their limited budget on upgrading an older car where safety retrofitting is not possible, or on migrating to a newer car.

You can't have a standard for a technology that doesn't exist yet. But anything that does have a standard can likely be repaired and upgraded more cheaply in the future. It preserves more of the value in the used cars.

If someone really needs those side-curtain air bags, they would be able to trade in their old vehicle, at a higher price, to someone who doesn't value safety features as highly, and use the proceeds to help pay for a newer vehicle with more of those features.