Servers shouldn't be 1:1 to begin with (this isn't the 20th century anymore and MMOs shouldn't be built that way in the 21st century). Server Worlds should be Clustered for Load-Balancing, High Availability, and Scaleability. At minimum, they should be 3:1 (three physical or virtual servers per Game World), and then scaled up according to Peak Capacity x 2 (generally you should plan to stress your servers to a max of only 50% capacity when at Peak). When a World begins to Peak past 50% capacity you add more servers to that Cluster until the Peak drops below or at 50% capacity. This is Systems Administration 101.

Some MMOs cap a game world at 4,000-10,000 players, adding new Clusters (Game Worlds) rather than expanding existing Clusters to keep Game Worlds from seeming too overpopulated. That works too as long as your existing Clusters can handle Peak Load x 2.

True Story:

The MMO Dark Age of Camelot (DAoC) was the successor to the once popular text based MUD (Multi User Dungeon, the precursor to MMOs) on GEnie and AOL. Mythic anticipated 10,000 users at Launch, but smartly planned their capacity for 30,000 users (3x expected capacity). Launch went smooth, the champagne was uncorked, at which point the Lead Developer and CIO of Mythic glances at the login numbers and says "Guys, we have a serious problem!" Turns out they had 50,000 people show up right at Launch (and 250,000 shortly thereafter). Even planning for 3x the capacity they Stress Tested for, they were still unable to handle that many players. So, what did they do? First thing, the CEO tried finding somewhere that could overnight them enough Servers on the weekend and bought every computer that was for sale at retail in the city on a Friday night, and while he was doing that the Lead Developer/CIO started making Image Backups of everyone's workstations (they had 25 Devs at the time) and putting those workstations immediately to work as temporary Servers to handle the sudden influx of players until new Servers arrived. It wasn't pretty from an IT stand-point, but it kept things running smoothly until they got 100 new Servers delivered to their Data Center, then installed & configured. During it all, no players noticed a thing. From everyone else's perspective it was a successful launch and DAoC went on to be awarded "Game of the Year" (the success of which caused Microsoft to steal their Source Code to launch their own duplicate MMO called "Mythica", and attracted EA Games to buy out Mythic).

The moral of this story is that success can either make or break your MMO. Having too many customers can be a mixed blessing if you are aren't ready to handle it or aren't quick to handle it in any way you can. I can totally understand Square Enix being surprised at the success of their Open Beta (when all of these problems first began), but they totally dropped the ball when faced with higher numbers of logins than they had planned for. Before Phase 4 of Beta was over they should have been scrambling to scale their Clusters (or if not Clustered to begin with to be Scaleable, they should have Clustered them at that point) in anticipation of Early Access and certainly in time for Official Launch. Considering that these things only came as an after-thought, when limiting capacity and then limiting concurrent logins were their Plan A and Plan B respectively, speaks very poorly about them, and casts serious doubts on whether they are capable of successfully running an MMO.

Especially considering that FFXIV:ARR was supposed to be an apology for the disaster that was FFXIV, you'd think they would have tried harder to not allow it to become as equal of a disaster. I guess some companies never learn. Fool me once Square Enix and shame on you! Fool me twice Square Enix and shame on me!