You are right that this is an issue for the insurance companies. The reason why some old cars are not insured is because they have been crashed and damaged enough to where it is no longer worth it for the insurance company to insure them. However, if someone were to try and insure a 2000 model car, you would probably have to have a few things done to it before the insurance company would even consider insuring it.
It is possible that there are some 2000 model cars that are still in good condition and would qualify for insurance in your country. However, most 2000 model cars are not worth insuring because they are old and broken down, which means they cannot be repaired and will likely require major work before they can be driven again. This means that if you were to obtain insurance on such a car, it would cost more than just purchasing any other car of equal value or higher value than your current vehicle — it would cost more money than buying a brand new car outright.
