How much did they spend in development and advertising before that 4 month period and how quickly are those earnings dropping?

That’s the calculation you need to look at, not earnings as if expenses prior to release was 0$.

It also gave us SusanExpress and gold farming. Heaven help us.

Not sure if you are referencing WoW, Diablo Immortal, or Blizzard in general, but gold farming has been around a while.

https://en.wikipedia.org/wiki/Gold_farming#History

Definitely. Talking about WoW in particular. It's less lucrative now, but they were everywhere a decade+ back. Burning Crusade to Lich King era. Spelling out the names of their websites with orchestrated corpse die-offs in big cities like Stormwind and Ogrimmar.