If your less-than-new car is still legally roadworthy, the fact that it has fewer airbags than a future vehicle standard would not, by itself, make it uninsurable or lead to a claim being denied. A rule for future vehicle production does not retrospectively rewrite the rule of every older car already on the road.
On top of that, comprehensive car insurance typically covers damage to your insured vehicle and the third party's damage. But injury to passengers is generally dealt with separately, including through the Road Accident Fund, if negligence and bodily injury are involved.
So, five years from now, if someone sitting in the back of an older car is injured in an accident, the question wouldn't really be: Did this car have the latest airbag setup?
The more important question would be whether the car was legally roadworthy, who or what caused the accident, what cover applies, and what the specific policy and liability wording is.
That said, insurers will likely adjust the underwriting, repair-costs or policy wording as newer safety standards come into force. More airbags and sensors may make cars safer, but they can also make accident repairs more complex and costly.