I suspect that right now, GW2's reluctance to add *additional* permanent zones has a lot to do with their roadmap moving forward relative to their business model. Adding a single new permanent map has a huge server/cost overhead.
By contrast, having temporary content, like Queen's Pavilion and Sanctum, uses a 'slot' they can keep reusing for content, which doesn't add any server overhead. It's quite clever, really. Basically, they know in advance what their server budget is, and can allocate overhead accordingly. All while providing fresh content to their fanbase, without having to keep adding server clusters for it since it's temporary. And aging content nobody plays doesn't sit there, wasting money. Granted, a lot of the basic 'fixed' zones in GW2 need some love to help make them more utilized, but even the deadest of dead zones in GW2 usually has someone in them doing something, just due to how the Personal Story drags new people about.
But adding a new permanent zone is a big deal, they need to ensure they have the continuing funds to cover it moving forward. That we got Southsun so early, feels almost like they splurged a little, or they had budgeted before launch one additional zone already, which was to be used for multiple things (Southsun Activity, etc).
Man, I wish I could see their internal roadmap.
