I think Blizz is underestimating the popularity that Classic will reach... by a lot. There are a whole slew of players that are planning to play classic (return or brand new) who don't currently play retail and weren't willing to pay for a sub just to reserve a name and play a couple day trial (stress test). I think that launch day will see a lot of demand but I think Blizz likely has the infrastructure to at least increase the number of layers to support all these players. They should, however, plan to create a number of new realms on launch day if the demand necessitates it.
I think @Stfuppercut is correct. While there are a number of guilds that are already stacked with players and headset on playing on a certain server, there are plenty of smaller groups that are planning on playing together who will be far more fluid that most of you seem to think. If you have a couple friends playing and only two of you log in with the others 2hours estimate in line then it's entirely possible that that group will instead decide to roll on a lower pop realm to be able to play together.
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