There is much debate within the IAM about regular re-tests and it is something that I, personally, would like to see introduced. The IAM does offer additional training and assessment for after the advanced test but I think RoSPA's grading system is very good. If you pass at Bronze level initially then there is an incentive to continue training and try again for a higher level.
Of course, just because a car has the red badge (or green in the case of motorcycles) doesn't mean the person driving is an advanced driver. We see how standards slip all the time. People who must have been proficient when they passed their DSA driving test get progressively worse as time goes by and there is no reason to think that advanced drivers are any different.
Regards Steve G