I think you are forgetting the fact that A). A lot of the top Red Mages were abusing a bug B). Most of the current content fights just so happen to have mechanics that allow RDM to still put out DPS close to their Skill ceiling. C). As other players learn fights and newer fights come out with less movement other jobs will shoot passed it in dps.

For example, assume the RDM is at 4400 dps with a possible max of 4500 if played perfectly. Now assume the Ninja is pulling 4200 dps on a fight he's not familiar with with a possible average of 10% increase in dps for optimizing their rotation to mechanics and lining things up. Now that ninja has a possible 4620 dps while the RDM could never go beyond 4500. At face value people may see RDM as doing more dps than ninja but that actually isn't the case, it's just that RDM has an easier time hitting that ceiling than other jobs who still actually have higher dps but while learning new fights that dps falls a bit.