Within the game people are embracing the second generation philosophy.
I'm sure Blizzard is taking everything into account not only variable numbers of players.
I'm sure, Blizzard is watching EVERYTHING for future products and sales. People will come and go. This is an old game, in fact so old people played it over and over until they couldn't stand it anymore. The classic version went a long time before it's first expansion that caused a lot of burnout. With this longevity of high numbers is almost useless.
This is important too..... It's "free" to Battle.net accounts .... how can this be monitored ?..... People will come and go at will. People will play this game simply because it's free and not care. Huge variable !
4.5 million spike initially will account for something.... But sustainment will be an arbitrary number. Blizzard knows that, deep down we know it !......... The 4.5 million spike proves it's successful, numbers after that will not count as much. And again that's not everything, not even close.
People are enjoying Classic World of Warcraft by a large scale..... Take that into account more !!!!
Enjoyment factor = money
What Blizzard decides to do next in impossible to predict. We can only have fun by guessing, even by the "self-appointed" mmorpg.com accountants.