Well partially because levels don't even exist.
They do. You need to distinguish between the SE marketing department branding and actual game functionality. They can call it whatever name they want, but it's levels. Lotro doesn't have hit points, except it does, but they're just called "morale" instead. WoW doesn't have mp, except it does, it's just called "mana" instead.
The armory system is largely a cosmetic change. It's still levels. Instead of being a level 75 rdm who can change jobs to a level 40 blm, you'll be a level 75 gladiator who can change weapon styles to be a level 40 pugilist.
I don't think there's gonna be a cap
I find that doubtful. While there are several asian rpgs fond of infinite or quasi infinite level caps, I don't think any SE game has ever used that system, and I doubt they'll start.
We'll likely see one of 2 systems. Either each individual weapon style will have its own cap and they can all be raised to max with no opportunity cost, like FFXI jobs, or there will be a sum cap and you must divide up levels between weapon styles somewhat similar to the higher parts of FFXIV crafting. I'm betting on the former rather than the later, but I'm sure opportunity costs will find their way into FFXIV in some form of character development.