Ok,

So you have a perfectly maintained 300E with brand new airbags at how much ever you paid per bag and someone pulls out in front of you and you hit them. Both bags deploy (just like the old ones would have) and your car is total loss. How easy do you think it will be to get the other guy's insurance company to pay you the $10K or so you think your car is worth because of all the new parts you have on it? While some people do succeed in doing this you would have to present a very well documented case on how your 300E was in such better condition than all the others on the road. The insurance company would still probably fight and use the "We can only pay market rate and are not responsible for reimbursing for repairs" route.

Front Airbags provide some additional crash protection but their help is limited in comparison to the seatbelts in most crashes. The more helpful airbags in my opinion are the side impact airbags because there is no latteral protection for the driver or passenger in a side impact. Side curtain airbags are great also for preventing cuts from broken glass and keeping your head from impacting the center pillar. But these airbags are only in recent cars.

Because Frontal bags provide some minimal protection and their cost would be astronomical I personally would be willing to risk the odds of the bag working after its intended lifespan. The 126 is the first "production" car in the world to offer an airbag. Recently Mercedes bought one of these cars from its original owner that was equipped with one of the optional airbags and crash tested it. The car and airbag were 24-25 years old and the whole car functioned perfectly in the accident as it was designed to.

On an older airbag the only thing I would be worried about is unintended deployment due to an electrical fault.

I have to go with the liability issue with the reason like someone else suggested. Even the used airbag market is kind of touchy. Paypal the online payment provider for the Ebay auction site does not want their service used to purchase used airbags. In our litigate first have common sense later society companies have to protect themselves. Lawsuits have affected many industries. One only has to look at the personal aircraft industry to see what the effects of lawsuits can do.