Even with a high concentration of players on two or three servers, I’m thinking Blizz will want to wait for live, release day data before opening more servers.
This is just knee-jerk thinking, and I have not a quintillion of logic to back up my belief, except a feeling that a flood tide of initial logins has got to be expected, and Blizz might simply and honestly want to see how the high population servers cope on opening day, and for the first week of live game play.
I also think that the long queues and wait times are only the initial, not the sole, frustration with which players who never played Vanilla will struggle to cope.
Because of motherboard issues, I did not start to play Vanilla until June 05, and the queues and wait times were often contentious and frustrating.