Personally, think this system works fine as is. The difficulty with scaling abilities lies in scaling player skill. The point of syncing is to maintain some semblance of balance. There is much great difficulty in attempting to balance a level-cap job with an at-level job. The at-level job has very few buttons to press. The level-cap job has very many (usually). Worse, the scaling would need to be balanced at nearly every single level, to account for new skills and traits. Every job gets that final piece of kit at level-cap, so balance to that, but at level-cap-minus-1, you have very different potency, needing a different scale. Additionally, you run the risk of having bad/mediocre level-cap players not fully utilizing their kit. If the scaling balance is meant to take into account a certain expected skill level, and players fail to meet it, you will find duty progress severely inhibited.

These reasons and more, in much more detail, always come up when this idea is presented. Bottom line, it's a considerable amount of work that constantly needs checked and rechecked, and it's just not remotely worth the effort when the current system achieves the balance goal with much less trouble.