I agree with you some of the old cars are well maintained than some newer vehicles. and also some of the newer vehicles from china and india are not having many safety features which many old european brands have,but vehicles are not meant forever, older it gets costlier it gets for repairs and quality will be deteriorating.
your point is 65- BMW even after deteriorating is still better than a new Maruti 800, maybe but it depends on the maintenance, therefore some 65- BMWs are very good some may not be,
This is the reason most of the countries they allow owners who still prefer to hold their old vehicle, can do that by paying a additional fee ( annually) fee will be increasing with the age of the vehicle.
There are many benefits for introducing a expiry date for vehicles,
1. Newer vehicles with better fuel efficiency will enter the market saving on forex to the country and less pollution
2. Prices of second hand cars will come down as, closer the expiry date for vehicle less demand it will have. vehicles will be properly depreciated. and more people can afford vehicles.
3. Create a balance in the vehicle population ( currently only accident condemned vehicles are remove from the roads)
4. When the local prices are low even used cars can be exported to countries without limits on age (africa)
My feeling is introducing a expiry for vehicles is a MUST for Sri Lanka.