The problem with that thinking is that Blizzard knows how much traffic is generated per logged in user per realm. With the 1.9 patch, they reduced the amount of traffic the game client sends in an attempt to reduce overall load on their end of the equation. This was clearly done to delay the need for infrastructure upgrades, be it out of no avaliability or financial considerations.
The problems they have created are twofold. The existing network infrastructure cannot handle the amount of traffic that is being pushed. This could have been avoided. They have had a year to say that Player X on average Y Kbps of traffic at any given time. Therefore, if they sign up, 2000 new players, they need to allow for a maximum of 2000Y Kbps of additional traffic. Granted, you'd assume that not all of these users would be on at any given time, but over 12 months you could have been able to determine the ratio of subscribed users to loggd in users, and adjust as needed.
They also have back end authentication issues. Auth waits have been more and more common in the past weeks, and if a scalable auth solution wasn't in place to start with, what were they thinking?
I don't blame the Blizz IT guys who are probable putting in 80 hour weeks. I blame management and sales who continue to push signups and sales without giving the IT guys time or resources to keep things running right.