We're in a different market, sure. The market where Diablo 3 sold more than 10 million copies. A much larger market.

Also, Diablo 3 can't be pirated - which was not the case with Diablo 2 and expansion. So, I'd expect an expansion to sell at least 7-8 million copies to make the base game qualify as a true success with fans. If people love it - they'll be more than willing to shell out for a meaty expansion.