GW2 elite specializations can do that because it's only 5 skills that change and you get a new themed trait tree with 15 traits.

FF14 jobs have closer to 20 skills that would need to be changed another 20 traits changed.

It's beneficial in GW2 because each character is locked to one class, expanding the weapons for the class increases repeatability and increases player options. In FF14 you have access to all jobs on each character so you already have the benefits of a weapon swap.

It's a great system for Gw2, but it doesn't convert well to ff14.