I may have been a little off with the nitrogen gas (very early technology). Modern 'tablets' use potassium nitrate to create the nitrogen gas. So we are talking totally solid form. So no gas at all in the tablet.
An interesting fact is that during WW2, airbags were experimented with to protect a pilot in the event of a crash. You can see why it was never implemented. A car crash and a plane crash are totally different ball games.
I have just discovered an article that indicates longevity of airbags. Apparently, in 1991, a 1973 Chevvy with an airbag was located. They connected the device to a battery and triggered it. It performed perfectly. So 18 years isn't bad for an airbag. I rekon they put best before dated on these to get more aftersales revenue.