Keep in mind it’s tens of millions of dollars of
recurring monthly revenue. While first month sales are inflated compared to the average, it would be just as notable if their monthly revenue were to fall below 8 digits in the months (or years) to come, and would be treated as evidence of a major blunder on Blizzard’s part. Unlike buy-to-play games, whose launch sales are the majority of its lifetime revenue.
Besides, it was never an either-or thing for Blizzard. They’re currently positioned to launch Diablo 4 while continuing to milk Immortal, and profit handsomely from both. The backlash from “core” gamers is only encouragement at this point, telling them that they’ve managed to tap a brand new market instead of cannibalizing their traditional business.