I oppose reducing its duration. Something about the nature of how FFXIV's netcode and general framework makes the timing for buffs finicky, even when you understand how it functions, and the flexibility to use it a GCD or two in advance helps assuage some of my grievances. Unless we drastically change the way things are applied to the status condition frame, and how they interact with other entities, tight timings with buffs or debuffs is annoying.

This does not address the knock-on effect for Arm's Length as a tank CD, though in the defence of your thesis there's a lot of benefit to decoupling actions that exist across different roles to give them fine-tuned versions, kind of like how casters use Surecast as it were. The slow on Arm's Length on physical DPS is a bit weird and the three roles that use it could benefit from their own secondary effects but I don't lose sleep over it.