Yoshi P has said he wants 3 tiers but only has the staff to do two. They are trying to hire more staff but skilled developers who speak Japanese and are willing to work in Japan aren't a dime a dozen.

I don't think this is a question of if they should or if they want to but if they currently can.

As for content release, FF14 is pretty damn competitive. Very few MMOs work on a mostly consistent 3.5 month patch cadence and guarantee new dungeons, multiple gearsets and some form of large scale group content (Alex/Mhachi stuff) in addition to other additions such as the side story stuff, the Primal fights, Deep Dungeons, and yes the Diadem stuff.