It's possible that SE learned from what happened with the Gold Saucer and will have plenty of extra, possibly temporary servers, to handle the load for certain zones - specifically, I'm thinking the first Ishgard zone you enter after passing through the Gates of Judgment. The way SE's got things set up, the FIRST THING anyone's going to do to experience anything in the expansion requires funneling through that ONE POINT. As soon as Early Access opens, everyone with early access is going to be lined up at the Gates of Judgment. That will be the first wave. As soon as official release happens, that's when the next wave will arrive. By making pretty much everything worth doing having passing through that gate as a requirement, they set themselves up for this, so I hope they're ready to deal with the consequences!

If they're not prepared... I see a strong possibility that it'll be difficult even to zone into Coerthas Central Highlands. The zone will be at capacity!

Early access will mitigate this somewhat, leading me to believe that they offered the feature as much to reduce their own strain as they did as a perk for preorders. Folks that enter Ishgard in the early access days will be able to clear some content and spread out to other zones, making room for others.