Consider: they have access to all of the sales data so they know how many copies of the game could potentially be played at launch.
They ran an open beta so they should have a good idea of how everything scales relative to total simultaneous user count.
They have extensive experience with all aspects of patching, scaling, and server operations through World of Warcraft and Starcraft II.
They intentionally decided to go with a "single player" experience that required connectivity and incurred server load.
Given all of that, there really is no excuse to fail as hard as they did on launch day. It is 2012, the standards are pretty high for getting things right with digital distribution and with online games. More so, if you make a bold decision to force connectivity for a single player game you damned sure better get it right or you are going to destroy your credibility.
Blizzard is enormously lucky that they have a very strong history of compelling games, these sorts of issues could easily cause an upstart game studio to go out of business.