Imagine for a moment if you will

[Nariel]

Honestly, you can get rid of class/job levels altogether and just have item levels. That keeps the sense of progression without the need to change/rebalance job each expansion due to increasing job levels (though you can still add or change skills as desired).

EDIT: As someone has pointed out:

Its not about raw damage input but rather refreshing the gameplay to keep it entertaining. Playing the same character with the same skill-set get stale fast. At least in the kind of MMO FFXIV is.

As an example, playing a mage that use the sames four spells but stronger for years don't feel like he progress as much as a mage learning new spell and new way to use the spells that got stronger.

[linay]

Its not about raw damage input but rather refreshing the gameplay to keep it entertaining. Playing the same character with the same skill-set get stale fast.

I was merely responding to the progression factor. Like I said, you can still modify skillset as desired for variability, but it's no longer tied to job level, which would necessitate it.